Powdered fuel conversion systems and methods

ABSTRACT

The burner preferably exclusively burns substantially explosible solid fuels and preferably has instant ON-OFF thermostat control, wastes no energy preheating the enclosure or external air supply, achieves stable combustion the moment the powder-air mix is ignited in our burner, is used in the upward vertical mode except for oil burner retrofits, burns a solid fuel in a single-phase regime as if it were a vaporized liquid or gas, is designed to complete combustion within the burner housing itself rather than in a large, high temperature furnace enclosure which it feeds, has an ultra-short residence time requirement, is a recycle consuming burner with self-contained management of initially unburned particles, is much smaller, simpler and lower cost, has a wider dynamic range/turndown ratio, is more efficient in combustion completeness and thermal efficiency, and operates with air-fuel mix approximately at the flame speed.

REFERENCE TO RELATED APPLICATIONS

This application claims one or more inventions which were disclosed inU.S. Provisional Application No. 61/042,996, filed Apr. 7, 2008,entitled “POWDERED FUEL CONVERSION SYSTEMS AND METHODS” and U.S.Provisional Application No. 61/074,244, filed Jun. 20, 2008, entitled“POWDERED FUEL CONVERSION SYSTEMS AND METHODS”. The benefit under 35 USC§119(e) of the United States provisional applications is hereby claimed,and the aforementioned applications are hereby incorporated herein byreference.

This is a continuation-in-part patent application of co-pending PCTPatent Application No. PCT/US2007/024044, entitled “POWDERED FUELS,DISPERSIONS THEREOF, AND COMBUSTION DEVICES RELATED THERETO”, filed Nov.16, 2007, which claims priority to U.S. Provisional Application No.60/859,779, filed Nov. 17, 2006, U.S. Provisional Application No.60/868,408, filed Dec. 4, 2006, and U.S. Provisional Application No.60/993,221, filed Sep. 10, 2007. The above-mentioned applications areherein incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention pertains to the field of solid fuel combustion. Moreparticularly, the invention pertains to sustained burning of explosiblebiomass powder with on/off control.

2. Description of Related Art

The present invention concerns processes, methods, devices, and systemsthat, taken separately and together, allow for the processing of biomassand other solid fuel materials into an explosible powder and thecombustion of the materials for a direct conversion into energy to heator perform work. This disclosure describes the harnessing of long-feareddust explosions and operating new solid fuel burners to accomplish aunique energy conversion process.

The present invention is largely based upon the application and newintegration of some advanced yet elegantly simple principles, portionsof which exist unconnected in various bodies of knowledge in the fieldsof fluid mechanics, physics, kinetics, industrial power plant processdesign, and combustion theory. This technology will soon gain integratedglobal scientific community attention and be applied in the engineeringof fuel source production, distribution, combustion burner design,heating, and other energy conversion applications.

A thorough discussion of the invention's background and practical andtheoretical bases is presented to convey the uniqueness of theinvention, the scope of its various embodiments and variations, and howit may be practiced. The present disclosure shows how prior art attemptsto utilize powder in fuels have come up short, failing to unveil apractical and complete picture of the methods and processes, which willsoon be established as a new body of knowledge and practice, becoming anaffordable and practical alternative to America's ever growing need forrenewable energy.

Before delving into the prior art, it is important to summarize keypoints about this new art and what performance and benefits can beexpected from its implementation. A burner of the present inventionpreferably has numerous features: instant cold start ON-OFF control;stable combustion the moment the powder-air mix is ignited; use ineither vertical and horizontal modes; burning solid fuel in asingle-phase mode as if it were a vaporized liquid or gas; completenessof combustion within the burner housing itself, rather than in a largehigh temperature furnace reactor; an ultra-short particle residence timerequirement; burning substantially explosible powders; recycle consumingwith self-contained management of initially unburned particles; andsmaller and simpler than prior art solid fuel systems. The burner andfuel in combination are important to operation of a burner of thepresent invention, as are the burner itself, the type and quality offuel, and integration with a Positive Displacement Powder Dispersion(PDPD).

A major point in our disclosure is the surprising revelation that asolid may be heterogeneously combusted in a gas in a method that differsvery little from a true single phase regime, yet differs greatly fromtraditional combustion practices over the years which continued to relyon two-phase principles of a stirred reactor. This topic will beintroduced in the review of prior art next, and explained in-depth laterin fluid mechanics terminology with reference to theory.

What has been the thinking, goals, and focus for design of burners,furnaces, and fuels over the last three to five decades, both in largepower plant burner, furnace and heat recovery design and fuel selection?It is apparent, after review of representative literature written duringthe last half century, that the basics of furnace design assumptionspracticed in the mid-twentieth century still control mainstreamthinking.

Residential and small commercial heating furnace design assumptions haveremained similarly bound and influenced by larger power plant concepts,except for changes in two significant areas. First, process control andenergy saving design improvements have resulted in increased efficiencyof heat recovery from small to large furnaces. Today, latent heat isextracted from hot flue gases with efficiencies in the low 90thpercentile normally. Second, using technology formerly only affordablein power plant furnace systems, these smaller furnaces and boilers arebeginning to experience technology additions to reduce airborne postcombustion pollutants, since it has not been cost-effective on a per BTUbasis or mandated outside the power plant.

Practicing new techniques of air pollution abatement have produced majorstrides forward by reducing, removing, and cleaning various pollutantsfrom power plant and furnace flue exhausts. Increased use of biomassbased fuels for co-firing with fossil fuels has further reduced stackemission levels. Ultra-clean coal, may soon become an affordable optionfor the residential and commercial users, but due to processing costs,has yet to become economically attractive for large coal firedgenerating stations.

The use of biomass for heating or transportation is often limited by ourexperiences as well, both on individual and governmental levels. We tendto think that alternatives to fuel oil must be liquids and fuels musttransport and pump like liquids. Likewise, supplements to gasoline mustbe liquid, except for wood gas and the hope for hydrogen.

Relevant Combustion History, Fuels, and Practices

There are several conditions which must exist simultaneously to achievecomplete combustion, known in the industry as the “Three T's”. The fuelmixture must be 1) in an environment of adequately high Temperature; 2)for a sufficiently long enough Time; 3) with reasonably Turbulent mixingconditions to provide proper oxidation to complete fuel combustion inthe Space allowed (see C. E. Baukal, Jr., ed., The John Zink CombustionHandbook), and that “Space” is known by a variety of names in theindustry such as a furnace, combustion chamber, boiler, firebox, andprocess heater (vertical cylindrical, cabin style and reactors), all ofwhich are large chambers or vessels emulating an “ideal mixed reactor”.It is important to remember that the primary method of heat transfer tothe fuel particles in such large furnaces is by radiation rather thanconduction from particle to gas as we employ.

Even back in 1950, furnace and burner design was driven by the goal ofattaining “ideally mixed reactions” as it states in the PlantEngineering Handbook (W. Staniar, ed.), incorporated by referenceherein. Design of a burner of the present invention must deal with thebulk these same criteria, but is not constrained to use of a model ofthe downsized power plant for furnace design, which requires a hot,radiating refractory and its inherent large size. As a benefit, ourburners can start up cold, and operate with ON/OFF control, unlike coalfired furnaces burning pulverized coal, which take hours to startup andshut down:

“The development of pulverized-coal firing for purposes of steamgeneration has been due, in large measure, to a better understanding offurnace design requirements. Uniform distribution of fuel and air to thefurnace is also of prime importance. Turbulence provides the means foreffective distribution and speed(s) ignition of the incoming fuel andpromotes rapid combustion by continually making available the freeoxygen needed by the ignited combustible matter. These requirements arethe governing factors in burner selection and application.”

“The selection [of a firing method] for any given installation isgoverned by a number of variables, of which the principal ones are size,shape, and volume of furnace available to develop the desired capacity.Furnace dimensions establish the maximum length of flame travelavailable . . . . Quantity of coal to be burned, as well as its volatilematter and sulfur content, fusion temperature of ash, and fineness ofpulverization will influence not only method of firing and type of wallconstruction to be used but also the method of ash disposal.”

“Each of the firing methods [vertical (downward) firing; horizontalturbulent firing; and tangential firing] requires a different burnerdesign because of the variations in the manner in which air and coal aremixed to produce efficient and complete combustion. Fundamentally,however, all burner designs must be such that the air and coal aresupplied to the furnace so as to provide stable and prompt ignition;positive adjustment and control of ignition point and flame shape,completeness of combustion; uniform distribution of excess air,temperature, and gas flow at furnace outlet; freedom from localized slagdeposits; protection against overheating, internal fires, and excessivewear in the burner; and accessibility for adjustment and replacement ofparts.”

Coal has been burned both in crushed and pulverized forms for over halfa century. There are many variables that interplay including moisture,percent volatiles, ash, and BTU value for given types of coal and thetype of furnace. For example, a high percent volatiles can cause heatingvalue loss or excess smoking issues for stoker-fired plants, if thespecific furnace has inadequate space and time to mix the volatile gaseswith air and completely combust them. A requirement for an upper limiton volatiles is a typical solution for specific types of furnaces.

Conversely, when firing pulverized coal, it is important “to set a lowerlimit (for percent volatiles) in order to maintain flame propagation,particularly in completely water-cooled furnaces” according to the PlantEngineering Handbook, page 373:

“Solid fuels, when burned in suspension, should contain an appreciablequantity of extremely fine dust so as to ensure prompt ignition. Theamount of coarser material must be minimized if best combustion resultsare to be obtained.”

“The fineness to which coals should be pulverized will depend on manyfactors. Caking coals (sulfur containing bituminous coal, coking coal,forms a fused heavy crust at the surface), when exposed to furnacetemperature, will swell and form lightweight, porous coke particles.They may float out of the furnace before they are completely burned. Asa result, carbon loss will be high unless pulverization is very fine.Free-burning coals (contains no sulfur and does not cake), on the otherhand, do not require the same degree of fineness because the swellingcharacteristic is absent.”

“High-volatile coals ignite more readily than those with low volatilecontent. Therefore, they do not require the same degree of finepulverization. With the exception of anthracite (called stone coal),however, the low-volatile coals are softer and may be said to have ahigher grindability.”

“Some large furnaces may operate satisfactorily on high-volatile coal ascoarse as 65 percent minus 200 mesh. Small water-cooled furnaces, usinglow-volatile coal, may require a fineness of 85 percent minimum through200 mesh. Other influencing factors are burner and furnace design,disposition of furnace volume, length of flame travel, furnacetemperature, and load characteristics. In general, however, smallfurnaces require finer pulverization than large ones.”

“The fineness of the product is usually expressed by the percentage ofdust that will pass a sieve with specific size openings. For testingpulverized coal the most commonly used sieves are the 50-mesh sieve (210microns) for determining the oversize and the 200-mesh sieve (74microns) for determining the fine dust.”

In summary, the use of pulverized coal in furnaces is most beneficialfor ignition, where an “appreciable quantity of extremely fine dust” isutilized. Pulverized coal offers benefits for handling with many typesof stokers but is a known detriment and avoided for caking types ofcoal. Coal raw material composition and resulting combustion issues canbe compensated for by adjusting the particle size to be finer orcoarser. The percentage of volatiles is similarly employed as atrade-off, with a higher volatile percentage enabling a distribution oflarger particle size coal to be used.

As evidenced by the art practices, pulverized coal particle size is usedto address fuel handling and coal type composition issues. By relying onthe % volatiles consumed in multi-phase combustion combined with largereactor size and residence time common to all furnaces, it is clear thatthere is no teaching in the art for using only substantially explosiblecoal powder as a feed stock with a cold, small and low-speed burnerdesign. The main value of having a portion of the overall pulverizedsize distribution well below 200 microns is for reliable ignition andfast burn only. Further in-depth fluid mechanics comparison of thedifferences between of our combustion regime and coal power plantfurnace techniques may be found later in this disclosure.

In traditional furnaces used for steam generation in power plants,whether they are fed by coarse crushed coal or fine pulverized coal,much of the actual combustion takes place over time inside the furnace'slarge volume. Typically, multiple burners are used to “fire” into theradiation filled furnace cavity reaction chamber, where much of thecombustion is completed and heat energy is released for subsequentexchange.

Biomass, Wood and Hog Fuel Combustion

Sources of biomass have been used sporadically in localized developmentsto convert accumulating “bio-scrap” for recovery of some of its energycontent and to “dispose” of this otherwise waste product. The pulp andpaper industry and affiliated sawmill industry are leading examples. Thefollowing gives perspective to the supply.

Biomass as Feedstock for a Bioenergy and Bioproducts Industry: TheTechnical Feasibility of a Billion-Ton Annual Supply, (R. D. Perlack etal.) sponsored by the U.S. Departments of Energy and Agriculture in 2005offers two significant quotes from the Executive Summary regarding theavailability of biomass as a fuel source:

“This study found that the combined forest and agriculture landresources have the potential of sustainably supplying much more thanone-third of the nation's current petroleum consumption.” And regardingdevelopment of a vertical industry of supply: “In the context of thetime required to scale up to a large-scale biorefinery industry, anannual biomass supply of more than 1.3 billion dry tons can beaccomplished with relatively modest changes in land use and agriculturaland forestry practices.”

Large “powder burners” from either Petrokraft or the VTS Powder burnerare utilized in Sweden and in Europe according to a 2004 doctoral thesiswritten by Susanne Paulrud, “Upgraded Biofuels—Effects of Quality onProcessing, Handling, Characteristics, Combustion and Ash melting”. Thistechnique is typically applied to large-scale heating plants over amegawatt. The fuel for these burners is “finely milled wood powder orfinely milled pellets.” Wood powder analyzed by sieve and laser methodsshows percentages of explosible particles ranging from 3% to about 46%,far too low to operate in the explosible mode. These burners utilizeclassic swirl for containment and recirculation mixing, but“aerodynamics and stoichiometry can make it difficult to achieve stableignition and good burnout”. Even with the finest particles, predictedparticle traces show distinct zones for evaporation, boiling, anddevolatilization before char burnout, indicative of two phasecombustion. This fact plus the large burner airflow designs and particledistributions used confirm no capability of operation mimicking a singlephase combustion regime.

Large burners such as the German burner utilized in the Canadian systemby Alternative Green Energy Systems Inc. (AGES), likewise consume woodparticles, sawdust and what they describe as powder as evidenced by thecomplexity, orientation, and ash concerns of their combustion equipment.This advanced system, however, was clearly not designed for exclusiveuse of a “substantially explosible” biomass wood powder.

U.S. Pat. No. 4,532,873, “SUSPENSION FIRING OF HOG FUEL, OTHER BIOMASSOR PEAT”, issued in 1985 to Rivers et al., is an excellent example ofthe previous and current art when it comes to the direct burning ofvarious types of biomass for heat recovery, in this case in a water-wallboiler.

While this hog fuel biomass burning system may initially seem verysimilar to our disclosure, detailed examination will make absolutelyclear that this entire system operates using a totally differentcombustion regime and substantially different operating principlesburner hardware, and fluid mechanic processes in its two-phaseoperation.

The patent states that the fines portion is an ignition source,imparting stability to the flame, and that “the presence of the finesportion is the heart of the invention” as it simply “eliminates therequirement for running with supplemental oil . . . . Hog fuels must besubstantially reduced in size to provide an ignition energy source”. Thelarge particle size distribution curve of FIG. 1 a depicts a typical hogfuel non-explosible particle size distribution, compared to anexplosible powder particle size curve on the left.

The stated stability of their two-phase combustion regime has only a2.5:1 turndown ratio compared to our 10:1 ratio, and their burner cannottolerate cold secondary air unlike a burner of the present invention.The process by Rivers, et al. is stated to work “for all furnaceconfigurations, kilns and the like, but is most particularly suitablefor use with water wall furnaces and boilers”. It relies on fines toinitiate and stabilize the combustion and radiant heat transfer from ahot furnace to complete it, especially when, large and oversizednon-explosible particles are concerned.

This hog fuel burner system requires a distribution with particle sizesmuch larger than ours, allowing for up to an estimated 75% of theparticles outside the explosible range (“15-85% less than 150 microns”)and “65 to 100% less than 1000 microns”, meaning 35% could be largerthan 1 millimeter (1000 microns), a size that is 4 to 5 times theboundary between explosible and non-explosible wood powders.

Even the slightly narrower region claimed by Rivers, et al. (“at least60% by weight of the particles are finer than about 1000 microns”)allows for a significant portion of non-explosible particles. Thestatement “A fines portion including at least 15% by weight less than150 microns was found suitable” clarifies that there is no requirementfor significant or substantial use of “fines”. The hog fuel burner doesnot operate in what we call the explosible range, a term they never use.Explosiblity is a phenomena they only understood from a standardindustry standpoint, for they were afraid of dust explosions like therest of industry, as made exceedingly clear by this last clarifyingstatement. “Fuels much finer than 85% less than 150 microns are likelyto be too ‘dusty’ increasing dust explosion hazards and otherwiserequiring an excess of pulverizing power to produce.”

The present disclosure focuses on combustion of substantially explosiblemixtures. Other art, including the hog fuel patent just described andco-firing designs, specify the use of a distribution of a mixed particlesize fuel, often called “powder”. Only some component segments of thebroad, larger fuel particle size distribution are “fine powders” thatmay, only when used alone, actually be explosible.

However, these “fine powder” portions are subsets of a much wider andessentially non-explosible fuel particle size distribution such as shownin FIG. 1 a, and are utilized at best simply as a quick and easilyburnable ignition and combustion maintenance energy source. This smallfraction has the stated primary purpose to “sustain combustion” oflarger particles and chunks, the major portion of their fuel sizedistribution lying outside of the explosible range, the region which wedisclose, claim, and prefer.

In the present disclosure, all of the fuel performs the functions ofignition and combustion temperature maintenance, not simply a portionnor even a significant portion of the overall fuel composite particlesize mixture. Substantially all of the fuel has the job of ignition andheating of its neighbors in the entire mixture, even “less burnable”agglomerated clumps or occasional, longer high aspect rationon-explosible particles having explosible diameters found in the fueldue to manufacturing sieving/separation imperfection.

In a case study begun in 1995, and entitled “Co-Combustion of Biomass inPulverised Coal-Fired Boilers in the Netherlands” (M. L. Beekes et al.),co-combustion of pulverized wood with pulverized coal was studied at theGelderland power station. Waste wood was used in a coal-fired boiler ina pulverized mode, as it “has the advantage of being a very dry and finefraction material that is uniform, easy to handle, and with high energycontent that can be burned much like oil or gas”. The 635-MWe coal-firedproduction began operation in 1981, and in the mid to late 1980's wasupgraded with flue gas desulphurization, NO_(x) reduction, andelectrostatic fly ash filters. Using four burners of 20 MWe each, thisbio-scrap can provide about 12.5% of the operating energy input.

Wood chip up to 3 cm in size were reduced by a hammermill at the plantto a maximum particle size of 4 mm. The particles were sieved anddivided and further separated using a dust collector. The particle sizedistribution of the wood powder is given as 90% less than 800 μm (acoarse 20 mesh), 99% less than 1000 μm, and 100% less than 1500 μm, witha moisture content of less than 8% by weight. With the materialdependent dividing line between explosible and non-explosible powderparticle size for wood residing in the neighborhood of 200+/− microns,it is obvious that a significant portion of the particles, likely wellover 50%, are not explosible, meaning their combustion process isdifferent from ours.

Combustion took place inside a boiler furnace built in the followingburner configuration: “Four special wood burners with a capacity of 20MWth each are mounted in the side walls of the boiler (two on each side)below the lowest rows of the existing 36 coal burners. There are 3 rowsof 6 coal burners in the front and back walls. The coal burners can alsobe used for burning oil and therefore the combination wood powder/oil istheoretically possible.”

Use of large furnaces as high temperature reactors allowed largerparticles as a substantial portion of the entire fuel stream to beutilized, as ignition happens inside the furnace through radiation, notparticle-to-gas conduction, characteristic of our single-phase appearingcombustion regime.

The “Safety Precautions” section of the report combined with theiroperating particle size specification and use with a large furnace makesit abundantly clear that this system was not operating in the explosiblerange and therefore not mimicking a single-phase regime as we practice,disclose, and claim. A byproduct of their “ . . . micronizing processcreates wood dust particles that pose a possible hazard for dustexplosions. Therefore [the following] safety precautions must be taken .. . . ” Like other recent prior art uses of biomass scrap for industrialenergy conversion, they too were unaware of the potential to operate inthe combustion regime of the present invention.

In the North American wood products and pellet industries today, it iscommon to see large cyclonic burners such as units manufactured by Onixused for chip and coarse sawdust drying. These Webb Burners™ are costlyand large in size to insure adequate residence time for particle charburnout. Suspension burners are taking over most installations, as theyoffer efficiency gains of 25% through stat gas recycle plus considerablyless maintenance. These large burners are designed for large particlefuel, and therefore do not operate using the principles of thisdisclosure.

The limitations of the prior art establish the need for systems andapproaches for the conversion of biomass and other solid fuel powdersdirectly into energy by such means and methods to afford ON-OFF controlof clean, dependable, and efficient combustion.

SUMMARY OF THE INVENTION

A burner system of the present invention operates based on the fluidmechanics driven process of a moving stream of a powdered fuel in anoxidizing gas, feeding a stable stationary deflagrating flame wave withon-off control. Balancing mass flow velocity of an explosible fueldispersion with deflagrating flame front wave velocity produces astationary and stable combustion front and zone. To aid in this balanceand improve stability, the burner system may include both active andpassive secondary air, which provides turbulent mixing, particlerecirculation, combustion zone support over a wide turndown range, andcombustion completion. A de-agglomerization system may be used to breakup agglomerate clumps of the powdered fuel to return them to anexplosible state.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A depicts schematically explosible and non-explosible particlesize distributions.

FIG. 1B shows an ideal particle size distribution and a more typicaldistribution for substantially explosible fuels.

FIG. 1C shows three different shapes of substantially explosible powderdistributions.

FIG. 2A shows schematically particle combustion time versus particlesize.

FIG. 2B shows schematically heat transfer rate part to gas versusparticle size.

FIG. 3 shows minimum required ignition energy and flame speed as afunction of fuel concentration for an explosible powdered fueldispersion in an oxidizing gas.

FIG. 4 shows graphically two-stage combustion of a stationarydeflagrating flame wave front at the interface of a moving stream ofpremixed solid particles in an oxidizer.

FIG. 5 shows basic combustion phenomena observed for two solid fuelparticle size distributions.

FIG. 6 shows fluid mechanical processes and phenomena observed for twosolid fuel particle size distributions.

FIG. 7 shows volatile organic compounds (VOC's) from a flue gasanalysis.

FIG. 8 shows schematically a burner in a first embodiment of the presentinvention.

FIG. 9 shows schematically a burner in a preferred embodiment of thepresent invention.

FIG. 10A shows schematically a horizontal explosible powder fueldispersion in an embodiment of the present invention with no flame.

FIG. 10B shows a magnified view of FIG. 10A.

FIG. 11 shows schematically velocity and gravity effects on a horizontalexplosible powder fuel dispersion with no flame.

FIG. 12A-D shows unconfined free space ignition at four ignition pointsin a horizontal explosible powder fuel dispersion.

FIG. 13 shows a first can embodiment of the present invention.

FIG. 14 shows a can embodiment of the present invention with secondaryair holes.

FIG. 15 shows a small can of the present invention with secondary airholes.

FIG. 16 shows stacked cans of the present invention with secondary airholes.

FIG. 17 shows stacked cans of the present invention with secondary airholes or slots and with a 1-inch gap between the cans.

FIG. 18A shows a first cut-away of a burner can with a sloped bottom andadjustable bottom air holes in an embodiment of the present invention.

FIG. 18B shows a second cut-away of the burner can of FIG. 18A.

FIG. 19A shows stacked cans with a multi-holed air ring supplyingsecondary air.

FIG. 19B shows a cut-away of the stacked cans of FIG. 19A.

FIG. 20A shows stacked cans with secondary air from a blower into lowerinlet holes.

FIG. 20B shows a cut-away of the stacked cans of FIG. 20A.

FIG. 21A shows stacked cans with three secondary air nozzles and a1-inch gap.

FIG. 21B shows a cut-away of the stacked cans of FIG. 20A with adeflagrating flame.

FIG. 22A shows a 6-inch stove pipe with no secondary air holes.

FIG. 22B shows simplified burner combustion basics for stove pipe ofFIG. 22A.

FIG. 23A shows a 6-inch stove pipe with secondary air holes.

FIG. 23B shows simplified burner combustion basics for stove pipe ofFIG. 23A.

FIG. 24A shows flame height for a low primary air flow rate with nosecondary air.

FIG. 24B shows flame height for a medium primary air flow rate with nosecondary air.

FIG. 24C shows flame height for a high primary air flow rate with nosecondary air.

FIG. 25A shows flame height for a low active secondary air flow rate.

FIG. 25B shows flame height for a medium active secondary air flow rate.

FIG. 25C shows flame height for a high active secondary air flow rate.

FIG. 26A shows a cut-away of a 6-inch stove pipe with copper secondaryair nozzles.

FIG. 26B shows a schematic view of fluid flow with a deflagrating flamefor the stove pipe.

FIG. 27 shows a 6-inch steel stove pipe burner with four copper activesecondary air tubes.

FIG. 28 shows agglomerate recirculation and mixing using a 30° coneinsert in a 6-inch stove pipe of the present invention.

FIG. 29A shows agglomerate recirculation and mixing using four secondaryair inlet holes and vibration in a 6-inch stove pipe of the presentinvention.

FIG. 29B is a magnified view of the lower right section of the burner ofFIG. 29A.

FIG. 30A shows a wide hole 30° cone combining both recirculation andsecondary air in an embodiment of the present invention.

FIG. 30B is a magnified view of the bottom section of the burner of FIG.30A.

FIG. 31A shows a burner with an ultrasonic agglomerate lump dispersingscreen system.

FIG. 31B is a top view of the ultrasonic agglomerate lump dispersingscreen of FIG. 31A.

FIG. 32 shows a burner of the present invention with a wide 30° cone,baffled passive secondary air, and ultrasonic lump destruction.

FIG. 33 shows a burner with a top hat flow reducer mounted at the top ofthe stove pipe.

FIG. 34A show a horizontal four-inch can burner of the presentinvention.

FIG. 34B is a cut-away view of the burner of FIG. 34A.

FIG. 35 shows the internal structure of a 4-inch horizontal burner ofthe present invention with dual coaxial enclosures for passive secondaryair management.

FIG. 36 shows a 4-inch horizontal burner of the present invention withfour active secondary air tubes.

FIG. 37A shows graphically a recycle collecting horizontal burner of thepresent invention, tilted slightly above horizontal for agglomerate andoversize particle collection.

FIG. 37B shows graphically a recycle collecting horizontal burner of thepresent invention, tilted slightly below horizontal for agglomerate andoversize particle collection.

FIG. 38 shows a more automated recycle consuming gravity collectingclosed loop solid fuel horizontal burner system of the presentinvention.

FIG. 39A shows a recycle agglomerate destructing horizontal burner ofthe present invention with an ultrasonic driven screen fordeagglomeration.

FIG. 39B shows an end view of the burner of FIG. 39A.

FIG. 40A shows piping drawing details of the mixing zone and infeed forhorizontal burners of the present invention.

FIG. 40B shows an ultrasonic deagglomeration screen in the mixing zoneto the piping interconnection for horizontal burners of the presentinvention.

FIG. 41 shows graphically upward vertical, horizontal and downwardvertical orientations for a solid fuel explosible powder burner of thepresent invention.

FIG. 42A depicts the internal structure of a large horizontal two-stageburner of the present invention with dual coaxial enclosures for activesecondary air management.

FIG. 42B adds first-stage active secondary air to the two-stage burnerin FIG. 42A.

FIG. 43 shows a block diagram of a complete furnace system utilizing anexplosible powder burner system and supply for heating.

DETAILED DESCRIPTION OF THE INVENTION

The invention described herein enables the sustained combustion ofexplosible powder mixed with an oxidizing gas in a dispersed travellingsuspension to produce heat or perform work. The combustion technologyand systems disclosed herein provide an opportunity for a majorreduction in dependence on fossil fuels, leveraging and utilizing localproduction and distribution of renewable biomass energy fuel to fill inthis gap, without the introduction of significant quantities of “new”CO₂ into the atmosphere.

No burner system to our knowledge is designed to exclusively handleexplosible biomass and other solid fuels suspended in a substantiallyexplosible mixture of an explosible powder with an oxidizing gas,preferably air, with the result of direct energy conversion, in a mannerthat mimics single-phase combustion of propane or methane gas.

We are the first to develop, use, and disclose an explosible powder as afuel in a burner, where the burner was designed around combusting asubstantially explosible solid powder travelling in a moving stream withthe process calculated to mimic a single-phase regime. We have developedand disclose the “Bunsen burner for solid powdered fuels”, then movedfar beyond in combustion efficiency control as we manage dustexplosions.

A significant feature and advantage of our burner disclosure is theincredible simplicity of the design. While integrating a number ofcomplex features, this simple design provides a direct conversion ofsustainable dried biomass and other powders in the form of asubstantially explosible powder. In this simple process disclosed inPCT/US2007/024044, using our newly disclosed burners, no additionalintermediate processes, consuming time, energy, and financial resourcesare required.

Because of our design strategy, a significant part of our claims is ourability to operate a solid fuel burner with substantially instantaneousON/OFF control, on demand. An additional unique feature is our abilityto modulate the burner output linearly over an operating dynamic rangein excess of 10:1, while maintaining our near-instantaneous ON/OFFcontrol capability. This modulation of a solid fuel burner output, witha fast response time of 1 to 2 seconds to a change in demand, is uniqueand unheard of for non-gaseous or non-liquid fuel source supply systemsand their respective burners. This combination of ON/OFF and linearoutput response control is unique for solid fuels, especially the widedynamic range achievable in a preferred burner configuration.

Our disclosure of means and methods to establish and maintain explosibleconditions for burning also represents additional discoveries. Ourability to deliver and burn solid powdered fuel, particularly biomass,with substantially 100% combustion completeness in a burner, is asurprising and unexpected discovery. The lack of observable or sensiblesoot and volatile by-products, combined with near-zero combustionresidue (left-over ash, slag, char, unburned particles) attests to thisfact. This unexpected benefit involves our choice of a combustion regimeafforded by burning a substantially explosible powder in an explosiblemix dispersion in our burners. This unique benefit is based on the fluidmechanics and kinetics of burning explosible, high surfacearea-to-volume ratio powders, which dictate the combustion processes forenergy conversion in this special region of gas-particle mixtures whichwe call explosible.

We have been able to reduce classic and long-practiced prior artconversion of solid fuels to energy, particularly biomass sources, froma cumbersome and inefficient multi-phase process to a substantiallysingle-phase combustion process, acquiring numerous benefits likesignificantly reduced size combustion devices and cold start capability.By being substantially single-phase in appearance, our processeliminates the time/space consuming necessity of vaporizing volatilesand later slowly consuming char, which dominate combustion of larger,non-explosible particles found in many so-called “powder” fueldistributions including some pulverized coals and wood based hog fuels.

Many will immediately ask why the fuels, combustion techniques, andresulting applications we disclose have not been discovered anddeveloped before. Some with a technical background in the art will beamazed with what is new, and the implications for an entirely newvertical industry, “from farm to flame”. Those skilled in the art willimmediately be able to build on and practice our inventions. While someold and current applications appear similar, in hindsight the uniquenessand simplicity of our novel and radically different approach to fuelproduction and energy conversion will be appreciated. The low levels offlue gas VOC's, with no noticeable soot, smell or smoke and thecompleteness of combustion offered by our disclosed powder fuels andconversion means is indeed surprising and unexpected.

A goal for each new burner design of the present invention is tomaximize the near-instantaneous combustion of all explosible particlesin the moving explosible dispersion stream with a stationarydeflagrating flame wave front, while minimizing the amount ofagglomerate, particles, and combustible gases (non-CO₂/H₂O) survivingthe initial pass that must be burned through recirculation, therebydriving substantially to zero any unburned particles remainingthereafter.

A preferred goal for various embodiments of our burner inventions is toestablish design and operating process parameters to achieve aheterogeneous combustion regime whereby we have instant ON/OFF and arange of 10× linear BTU/hour control, with the combustion processsubstantially taking place inside our cylindrical stovepipe burners,enclosures that are surprisingly small relative to prior art.

As detailed elsewhere in our disclosure, the vast majority of combustioncompletion occurs within the confines of an upward firing verticalburner enclosure, mimicking a single-phase process. For example, whencombined with a typical industrial or home forced air (or hot water)type of furnace, the host appliance provides only a system for heatexchange and flue gas handling and heat recovery, plus furnace processand environmental control. When our burners are integrated with suchfurnaces, there is no sharing of the combustion functions. The sameholds true for near horizontal and below horizontal burnerimplementations of the same basic technology, commonly used for oilfired furnaces.

Our discovery of near-instantaneous combustion of all explosibleparticles in a small and initially cold burner is a surprising andinventive improvement over comparatively slower combustion prior artpracticed in burning hog fuel (see U.S. Pat. No. 4,532,873), currentwood chip furnaces with other biomass fuels, as well as pulverized coal(see M. L. Beekes et al.). Typical biomass-based fuels containsubstantial portions of larger particulate and sometimes non-explosiblepowder. To achieve char burnout for complete combustion, a longresidence time is usually required in the combustion zone, since theoverall mass of larger, oversized particles burn slower, even when“seeded” with powder sawdust or dust in the explosible range.

To operate a burner combustion devices of the present invention, we relyon the use of a substantially explosible powder as the total energysource for conversion. This fuel particle size distribution with itsupper limit near the explosible diameter for a given fuel sourcematerial is the heart of our disclosure in PCT Application No.PCT/US2007/024044. This unique fuel provides the motivation fordevelopment of a burner combustion system tailored to work with suchspecific explosible distributions to achieve the surprising capabilityof substantially 100% complete combustion of a solid fuel, using methodsthat make it behave as if it were a gas operating in a single phaseregime.

These inventive techniques are scalable to design burner and PositiveDisplacement Powder Dispersion (PDPD) system components and combinationsto produce a flame from the size of a candle to a megawatt power stationburner, yet with substantial apparatus design and configurationdifferences from current fuel and burner technology art. A primary, butnot limiting, focus is small and medium scale residential and commercialplus industrial applications of this energy conversion technology toreduce our dependence on import and utilization of foreignhydrocarbon-based fuels.

A complete burner-PDPD combination of the present invention isconvertible and adaptable to various explosible powders, each with theirown calorific energy and explosible particle size limit. Tests have beenrun on flour, pancake mix, confectioner's sugar, and corn starch as wellas particle size distributions of various hardwood (74 micron called 200mesh) and softwood (177 micron called 80 mesh) explosible powders.Bio-scrap such as corn stalks, and field grass, hay, and various woodybiomass have also been reduced in test grinding operations using a widerange of commercially-available particle reduction apparatuses andcombusted in our burners.

Combustion systems of the present invention have a dynamic range ofoperation (turn-down ratio) far exceeding that typically found in priorart commercial and industrial burners utilizing biomass products with agreat similarity to the adjustability of propane (LP) and natural gasburners found in the common stove.

The technology and combustion regime practiced and disclosure herein isvery different from current commercially available burner/furnacecombinations that use wood chips and/or powder as a sole source or inco-firing applications. Emissions are ultra low, with no visible sootand no discernable odor.

Focused use of a substantially explosible powder combined with thevarious burner embodiments in this disclosure allows for attainment ofnear 100% complete combustion. No significant burner or furnaceresidence time is required for complete combustion of particles. Nosignificant ash is produced, hence grates common to solid fuel furnacesare not required and regular cleanout of unburned char is not necessary.Staged combustion is not required but may be preferred. We utilizecombustion process models very different from many conveyor- andauger-fed furnaces, where substantially all the fuel is burned by twophase combustion, in large, high-temperature, environment reactors whereradiation is the primary source of particle ignition and heating.

A focus is to utilize powder forms of biomass including, but not limitedto, corn stalks, grass, sawdust, bamboo, wood chips, andchemically-cleaned, ultra-clean coal, as a direct replacement for liquidand gaseous fuels. In our PCT application PCT/US2007/024044, wedisclosed the requirements for these powdered fuels to be explosible andprocess methods of combustion by a deflagrating flame. The presentdisclosure reaffirms and relies upon material contained in thatapplication, and further discloses a number of methods, systems, andburner apparatuses to perform combustion, thus enabling the use of morepowder fuels to provide energy for heat or to perform work.

No one has previously invented the type of combustor/burner system wedisclose, because there has been no awareness of the controllable natureof the burning principles required to harness dust explosions, andessentially no supply of the proper explosible powder fuel supply. Wedisclose a radically new type of burner-PDPD system, designed anddeveloped for “the exclusive use of a substantially explosible powder”continuously supplied and dispersed, then combusted in a substantiallyexplosible mixture.

The goal-directed mindset of harnessing dangerous and therefore fearedand avoided dust explosions, combined with the development approachused, delivered us to the point of a series of discoveries that enabledthis invention. The combination of our investigative mindset and ourprototype development approach is based on unique “out of the box”thinking, that freed us from the very constructs which have bound manyother combustion investigators, researchers, scientists, engineers, andinventors for the last several decades, leaving our foundationaltechnology essentially undiscovered and unappreciated until our workbegan.

Burners of the present invention, with a range of embodiments andpreferably designed for use with an explosible powder combusted in anessentially single phase mode, are surprisingly small and simple. Intotal, the present invention provides a missing piece to the puzzle ofbiomass energy harvesting and conversion.

The present invention provides methods, systems, and apparatus for oneskilled in the art to assemble and utilize this new technology of“harnessing dust explosions” as a means of direct, efficient, and lowcost energy conversion of solid fuels.

A burner system of the present invention has not been previously known,since there had been no design nor integration combining the criticalcombustion principles and carburetion techniques for flow control anddispersion with solid fuel burner designs. No significant supply of theproper explosible powder fuel exists either.

In contrast to coal furnaces, as detailed herein, the vast majority ofcombustion completion occurs within the confines of an upward firingvertical burner enclosure in one embodiment of the present invention,mimicking a single-phase process. For example, when combined with atypical industrial or home forced hot air, forced hot water, or forcedhot steam type of furnace, the host unit provides only a system for heatexchange and flue gas handling, in essence heat recovery, plus processand environmental control. When a burner of the present invention isintegrated with such furnaces, sharing of the combustion functions isnot necessary.

DEFINITIONS OF TERMS

Before further description of the present invention, and in order thatthe invention may be more readily understood, certain terms have beenfirst defined and collected here for convenience.

The term “agglomerates” as used herein describes large, non-explosibleparticles of varying sizes and shapes comprised of numerous smallparticles self-adhering due to mechanical shear and other factors.

The term “air” as used herein describes a mixture of gases containingfree oxygen and able to promote or support combustion.

The term “biomass” as used herein describes any organic matter availableon a renewable or recurring basis, i.e. complex materials composedprimarily of carbon, hydrogen, and oxygen that have been created bymetabolic activity of living organisms. Biomass may include a widevariety of substances including, but not limited to, agriculturalresidues, such as grasses, nut hulls, oat hulls, corn stover, sugarcane, and wheat straw, energy crops, such as grasses including but notlimited to pampas grass, willows, hybrid poplars, maple, sycamore,switch grass, and other prairie grasses, animal waste from animals, suchas fowl, bovine, and horses, sewage sludge, hardwood or softwoodresidues from industries such as logging, milling, woodworking,construction, and manufacturing, and food products such as sugars andcorn starch.

The term “blended powdered fuel” as used herein describes a powderedfuel that comprises two or more distinct powdered fuels, each of whichmay vary in particle size, material, or composition.

The term “burner” as used herein is generic to “burner assembly”, and“flame holder” and describes a device by which fluent or pulverized fuelis passed to a combustion space where it burns to produce aself-supporting flame. A burner includes means for feeding air that arearranged in immediate connection with a fuel feeding conduit, forexample concentric with it. In patent documents the expression “burner”is often used instead of “combustion apparatus” and not in therestricted meaning above.

The term “burner assembly” as used herein describes a unitary device orfixture, including a flame holder and associated feeding or supportingelements.

The term “char” as used herein describes the mostly carbon solid residuethat remains when biomass volatiles are driven off during pyrolysis.

The terms “combustion” and “combust” as used herein, without referenceto a type of device, i.e., a combustion device, describe the act ofdeflagration. These terms are distinguishable from the act of simpleburning, which is the direct combination of oxygen gas and a burnablesubstance.

The term “combustion area” as used herein describes a location wherecombustion occurs, for example, adjacent to a nozzle or inside an enginecylinder.

The term “combustion chamber” as used herein describes a chamber inwhich fuel is burned to establish a self-supporting fire or flame frontand which surrounds that fire or flame. See also combustor and burner.

The term “combustion device” as used herein describes any system thatburns or deflagrates a fuel of any type. Such combustion devices includeinternal combustion engines, furnaces, grain dryers, and generators.

The term “combustion gases” as used herein describes the exhaust gasesproduced by burning the fuel, including chemical reaction products (e.g.CO₂, H₂O, NO_(x), SO_(x)), water vapor, and the non-reacting aircomponents (e.g. N₂). VOC and TOC are ratings used by the EPA.

The term “combustor” as used herein describes a combustion chamber withan igniter. While most of the traditional dictionary sources definecombustor in the context of jet engines or gas turbines, papers,reports, and products developed by those practicing in the art refer toburners combining a fuel, an igniter, and an oxidizing gas inside acombustion chamber as a combustor.

The term “combustion zone” as used herein describes the part of anapparatus where the reaction takes place between air and fuel.

The term “complete combustion” as used herein describes a combustionreaction in which the oxidizer consumes the fuel, producing a limitednumber of products. As such, complete combustion of a hydrocarbon inoxygen yields carbon dioxide and water. Complete combustion of ahydrocarbon or any fuel in air also yields nitrogen.

The term “controlled stream” as used herein describes a movement orstream of particles that may be directly controlled and modified, e.g.,by feedback modification, based on parameters flow rate, mass transferrates, power or heat output, temperature regulation, and the like. Thestream may be finely or coarsely controlled as the particularapplication may require. Moreover, devices, such as sensors describedherein below, may be used to provide the data necessary to control ormodify the stream. In particular embodiments, the stream may becontrolled for the purpose of producing a uniform explosible powderdispersion.

The term “deagglomeration” as used herein describes the act of breakingup or removing large particles comprised of groups of smaller particlesself-adhering in clumps.

The terms “deflagrating” and “deflagration” as used herein describerapid burning with intense heat output and possible sparks in a subsoniccombustion that usually propagates through thermal conductivity, e.g.,the combusting material heats the next layer of cold material andignites it. It should be understood that deflagration is distinguishedfrom detonation which is supersonic and propagates through shockcompression.

The term “devolatization” as used herein describes the releasing ofcombustible volatiles and tar from solid wood or other biomass orcombustible fuel during heating and is used interchangeably herein withthe term “pyrolysis”.

The term “equivalence ratio” as used herein describes the ratio of theactual ratio of the explosible powdered fuel to the oxidizing gas to thestoichiometric ratio of the explosible powdered fuel to the oxidizinggas.

The term “explosible” as used herein describes a property of a powder,which, when dispersed under the appropriate conditions as apowder-oxidizing gas mixture, is capable of deflagrating flamepropagation after ignition. Explosible powders that form explosiblepowder dispersions are capable of flame propagation when mixed with theappropriate ratio of an oxidizing gas. Numerous explosible powders,which are distinguishable from “explosive” or ignitable powders, aredescribed in Table A.1 of Dust Explosions in the Process Industry (R. K.Eckhoff).

The term “gas” as used herein describes any substance in the gaseousstate of matter, which contains a minimum amount of an oxidizing gas,e.g., O₂, to produce an explosible powder dispersion, even ifinsufficient to provide complete combustion, and is used interchangeablyherein with the term “oxidizing gas”. This term is intended to encompassgases of singular composition, e.g., O₂, and mixtures of gases, such asair. This is in contrast to the use of this term as the abbreviated formof the word gasoline, liquefied petroleum gas, or natural gas.

The term “heat exchanger” as used herein describes a device to transferthermal energy from the hot exhaust gases to a heat transfer fluid thatcan be water, air, thermal oil, or an antifreeze solution in acombustion system.

The term “heterogeneous combustion” as used herein describes combustionwhere the two reactants initially exist in different phases, whethergas-liquid, liquid-solid, or solid-gas. Heterogeneous combustiondescribes a solid particle oxidizing at its surface.

The term “homogeneous combustion” as used herein describes combustionwhere both reactants exist in the same fluid phase, either gas orliquid.

The term “incomplete combustion” as used herein describes a combustionreaction in which a fuel is incompletely consumed by the combustion.Incomplete combustion produces large amounts of byproducts. For example,incomplete combustion of hydrocarbons may produce carbon monoxide, purecarbon in the form of soot or ash, and various other compounds such asnitrogen oxides.

The term “particle size” as used herein describes the size of aparticle, e.g., in terms of what size mesh screen the particle will passthrough or by metric description of the size (e.g., in microns).Moreover, certain embodiments of the powdered fuel are defined, in part,by particle size. Particle size may be defined by mesh scales, in whichlarger numbers indicate smaller particles.

The term “particle size distribution” as used herein describes theprevalence of particles of various size ranges, i.e., the distributionof the particles of various sizes, within a powder sample.

The term “particulate” as used herein describes very fine solidparticles, typically ash plus unburned carbon that are entrained by thecombustion gases and escape to atmosphere. Usually the main airpollutant from biomass combustion.

The term “positive displacement” as used herein describes a techniqueusing devices that move a known volume of material per unit operation asin per stroke, per index, or per unit time.

The term “powder” as used herein describes a solid compound composed ofa number of fine particles that may flow freely when shaken or tilted.The powder composition, particulate size, or particulate sizedistribution may be selected based on the application in which thepowder is being used. “Powdered” is a substance that has been reduced toa powder.

The term “powdered fuel” as used herein describes a combustible solidfuel, reduced in mean particle size to a point where the substantialmajority of particles are below its particular explosible threshold andis used interchangeably herein with the terms “explosible powder”,“powder”, and “fuel”.

The term “powdered fuel dispersion” as used herein describessubstantially uniform mixtures of powdered fuel and an oxidizing gas,which are selected to be explosible based on the nature of the powder(e.g., size or composition of the constituent particles) and the ratioof the powder to the oxidizing gas and used interchangeably herein withthe term “powder dispersion”. The explosibility of the powdered fueldispersion may be affected by a number of factors including, forexample, the surface area of the powder particles, the energy content ofthe powder, the concentration of an oxidizer such as oxygen in thepowder dispersion, the temperature of the powder and the oxidizer, theheat transfer rate, and the powder particle size. The terms “powderedfuel dispersion” and “powder dispersion” are also intended to coverthose dispersions that include an imperfectly distributed mixture madewith an imperfect distribution of an explosible powder, provided thatsuch dispersions are explosible.

The term “pyrolysis” as used herein describes the thermal decompositionof organic fuels (e.g., biomass resources, coal, and plastics) intovolatile compounds (e.g., gases and bio-oil) and solids (chars) in theabsence of oxygen and usually water. Types of pyrolysis aredifferentiated by the temperature, pressure, and residence (processing)time of the fuel, which determines the types of reactions that dominatethe process and the mix of products produced. Slow (conventional)pyrolysis is characterized by slow heating rates (0.1 to 2° C. persecond), low prevailing temperatures (around 500° C.), and lengthy gas(>5 seconds) and solids (minutes to days) residence times. Flashpyrolysis is characterized by moderate temperatures (400-600° C.), rapidheating rates (>2° C. per second), and short gas residence times (<2seconds). Fast pyrolysis (thermolysis, using the fast pyrolysisexperiment of Nunn et al., 1985) involves rapid heating rates (200 to105° C. per second), prevailing temperatures usually in excess of 550°C., and short residence times. Currently, most of the interest inpyrolysis focuses on fast pyrolysis because the products formed are moresimilar to fossil fuels currently used.

The term “secondary air” as used herein describes air supplied to thecombustible gases liberated by the primary air in order to completetheir combustion. The term secondary air includes tertiary and higherorder airs.

The term “single-phase combustion” as used herein describes combustionwhere fluid mechanics single temperature and single velocity assumptionscan be made. Gases such as propane and methane burn in a single phaseregime, whereas gasoline and wood chip combustion is inherently twophase, liquid-gas and solid-gas respectively. Explosible powders,including sufficiently small particles under the proper circumstancesand mixed at the molecular level, burn indistinguishably from and as ifthey were gases in a single-phase regime.

The term “stoichiometric” as used herein, for example in “stoichiometriccombustion” or “stoichiometric mixture”, describes the ratio of theexplosible powdered fuel to the oxidizing gas in the powderedfuel/oxidizing gas mixture, i.e., a powdered fuel dispersion of theinvention, that is suitable to support deflagration and substantiallyconsume the explosible powder in the mixture or dispersion. Thestoichiometric amount of oxidizing gas necessary to consume theexplosible powder in the combustion area may be distinguished from theamount of oxidizing gas of the powder dispersion, which is sufficient tocreate an explosible mixture yet is typically lower than the totalamount of oxidizing gas that is ultimately capable of consuming thepowder. As such, powders of the present invention may be explosible evenwithout a stoichiometric amount of an oxidizer.

The term “turbulent flow” as used herein describes fluid flow having thefollowing characteristics: three-dimensional irregularity, diffusivityas in mixing, a large Reynolds number, dissipative in turning kineticenergy into heat, and continuum where the smallest scales are muchlarger than molecular scale, and is a property of the flow, not thefluid.

The term “turn-down ratio” as used herein describes a numeric ratiorepresenting highest and lowest effective system capacity. Turn-downratio is calculated by dividing the maximum system output by the minimumoutput at which steady, controlled, efficient, pollution-free combustionis sustainable. For example, a 4:1 turn-down indicates that minimumoperating capacity is one-quarter of the maximum.

The term “turbulent combustion” as used herein describes a combustioncharacterized by turbulent flows. In certain embodiments of theinvention the deflagrating combustion is turbulent combustion, whichassists in the mixing process between the fuel and oxidizer.

The term “volatiles” as used herein describes organic vapors and gasesreleased from biomass during low temperature heating, including thatportion of biofuels that is converted to vapors and gases duringpyrolysis, i.e. all components other than residual char. “Volatile mass”as used herein describes the mass of the powder fuel particles thatincludes material or compounds, such as water, which vaporize orvolatilize at or below the combustion temperature of the powdered fuel.

Basic Concepts about Explosible Particles

Basic combustion is rooted in fluid mechanics theory and other science.A powdered biomass fuel of the present invention burns like a gas forthe following simplified fluid mechanics based reasons: 1) The timescale over which a particle (solid or liquid) interacts with thesurrounding gas phase scales with R where R is a length scale (radius)of the particle. 2) As R decreases, the time required for particles toreach equilibrium with the surrounding gas phase goes down with thevalue of R.

3) As the particle R further decreases, the thermal equilibrium timebecomes small compared to the time required for other processes such asdiffusion and devolatization to occur. 4) At that critical particleradius R and below, a mixture of those particles and air is “explosible”under some definable conditions. 5) The explosible mixture can be dealtwith mathematically as if a mixture of two gases, because a single-phaseapproximation works.

The following points form a technical preamble and introduction to theuse of substantially explosible powdered fuels. 1) A powder-air mix is“explosible” when it supports combustion as a wave process, rather thanmore common burning in an ideally mixed reactor (i.e. furnace).

2) Explosible combustion of a solid fuel granular-air mixture happenswhen a particle's heating and combustion time is close to the timerequired for the passage of a combustion wave, and the reaction energyis such that the combustion energy released in the wave continues to besufficient to raise the temperature of the adjacent zone of unburnedfuel-air mixture above its ignition temperature.

3) A granular solid fuel dispersion in an oxidizing suspension is“explosible” when the particles are small enough forsingle-phase/single-velocity/single-temperature fluid mechanicsapproximations to accurately describe its behavior, and the dispersionis presented for combustion at an ignitable, hence explosibleconcentration.

4) The dynamic relaxation time for a single particle in a gas scaleswith R². As the particle diameter decreases below its material specificexplosible size upper limit, the difference between the two-phasebehavior of a solid particle-gas mixture and a single-phase gas-gasmixture disappears.

5) An “explosible” solid fuel powder behaves indistinguishably fromgaseous or liquid fuels under proper conditions. A granular solid fuel,when dispersed in an air suspension, can be made to move as a gas andbehave in combustion as a “pseudogas”, all without actually being one.

6) Any biomass or chemical solid fuel source, can, by reduction to aparticle size below its specific critical value, be considered an“explosible” powder. The cost of reducing a solid fuel from anon-explosible form, to a particle size that renders it “explosible”, issmall compared to the cost to convert it to a real liquid or gaseousfuel.

The main foundational technology of our discoveries falls in a gray zonebetween three large bodies of knowledge, with fluid mechanics, kinetics,and dynamics on one corner of this unstudied and therefore unreportedabyss, industrial dust explosions on another, and the engineering designof combustion systems including burners, boilers, furnaces, and otherheat using/producing equipment on the third.

The integrated technology disclosed herein is neither directly addressednor covered by any of the three. While we have gained confirmation ofthe science behind our discoveries and furthered our understanding fromthe theoretic and practical experiences available from each body ofknowledge, none has predicted our inventions.

Many of the experts that write about industrial dust explosions are notnecessarily well versed in the fields of fluid mechanics and combustiondynamics theory, but their research into dust explosions and mitigationis useful to some topics in this disclosure.

The particle distribution density in an oxidizing gas that supportsdangerous dust explosions is a range of concentration spanning more thantwo orders of magnitude, from 50 to 100 g/m³ to 2 to 3 kg/m³. WhileEckhoff (Dust Explosions in the Process Industries, 3^(rd) Edition, RolfK. Eckhoff, 2003, Elsevier, hereby incorporated by reference) describesthis range as “quite narrow”, it actually defines a wide controllableprocess range for our invention, whereby we perform energy conversion ofa dust-like explosible powder with a stationary deflagrating flamewavefront balanced with and surrounding a moving, premixed explosiblepowdered fuel dispersion in a burner.

In “The [Popular] Science of a Grain Dust Explosion” (R. K. Eckhoff) thecritical parameter is grain dust particle sizes at 0.1 mm or below. Asthe particle size decreases, the risk of an explosion increases.Concentration contributes to the dust's flammability and must be between40 grams per cubic meter and 4000 grams per cubic meter according tothis source. The actual limits vary based upon particle size,composition plus temperature and humidity, and may differ slightly fromEckhoff's reported range. Also, the dust must be in suspension, not justaccumulated in layers, for an explosion to occur.

Serious damage occurs only when there are both primary and secondarytypes of dust explosions. An ignition source initiates the primaryexplosion, producing a shock or blast wave that propagates throughout anarea loaded with dust layers, suddenly raising this large supply of idledust into air suspension. This highly-explosible fuel-rich suspensionmay be ignited within microseconds by the primary dust flame, and theresults are catastrophic.

Fluid Mechanics Background to Combustion of Explosible Powders

This section provides a basic description of combustion of an explosiblepowder with emphasis on deflagration and flame speed. In contrast to adust explosion, where the dust is stationary in a confined space priorto ignition and the flame wave moves during the explosion, in a burnersystem of the present invention, the fuel dispersion is moving to anopen space and the flame is stationary. A flame is produced when aflammable fuel source, an oxidizer, and a high-temperature environment,such as an ignition source of the present invention, are all present. Aslong as the three components are present, the flame will continueindefinitely.

From a fluid mechanics standpoint, in the present invention thedeflagration of substantially explosible powders is built on a portionof theory that fluid mechanics scholars and practitioners have not yetexplained as a whole. While we are not exploring a previously-unknownbranch of fluid mechanics, there is no one body of knowledge as of yetdescribing our unanticipated discovery concerning the application ofthese phenomena for the purpose of ON-OFF controllable energy conversionin fluid mechanic, kinetics, combustion, or heat transfer terminologyand theory.

It is indeed ironic that this region of operation has been avoided byvirtue of its most important phenomenon, dust explosibility. The presentinventors have discovered amazing benefits from the generous andforgiving properties of combustion in what fluid mechanics callssingle-phase combustion of solids. Specifically, while operating in aprofoundly single-phase mode, premixed explosible powders have a farwider operating range in terms of stoichiometry, than do the commonlyknown fuel gases such as propane, methane, and gasoline to name a few.

When a Positive Displacement Powder Dispersion (PDPD) feed system of thepresent invention disperses an explosible solid fuel, suspended in anoxidizing gas, horizontally into any open space (see FIGS. 10A, 10B, and11), this moving stream, which is initially well above stoichiometric,begins to slow and diverge, becoming ready for ignition. An arc igniteror propane torch flame instantly ignites this explosible mixture with anoticeable “whump” sound a few inches downstream of the PDPD feed nozzleas shown in FIG. 12A.

This unconfined and stationary flame front immediately consumes unburnedsolid fuel particles in the moving stream, travelling “upstream” at anear equal and opposite velocity of about half a meter to a few metersper second. If the velocity of the flame front remains highly subsonic,this combustion phenomenon is called deflagration, which means “to burnrapidly”. In certain process flow and confinement situations, the flamepropagation velocity may increase to near or beyond the speed of sound,producing detonation, a very intense explosion resulting in shock wavegeneration in the air-fuel mixture and surrounding area. This is not acommon event in such an unconfined environment, as a local, stationarydeflagrating flame is our method of combustion in this moving stream,travelling at the flame speed but never far from the ignition zone.

A lot can be understood about the process by first discussingdeflagration. Burning rapidly, deflagrating flames are produced bychemical reactions between “very finely divided fuel and oxidizerparticles”. The speed with which a deflagrating flame moves through anear-stoichiometric mixture is related to the fuel ignition temperature,its calorific value and particularly to the particle or grain size ofboth the fuel and oxidizer, which may be in solid, gas, or liquidstates. Biomass fuels ground to be explosible powders have asurprisingly high calorific output. In general, it is safe to say thatthe greater the uniformity of this mixture and the finer the fuelexplosible powder is ground (up to a point below 80 μm), the faster theflame speed at which it burns.

From a fluid mechanics perspective, balancing the mass flow velocity ofthe premixed explosible fuel with the deflagrating flame front/wavevelocity produces a stationary and stable combustion front and reactionzone. Central issues here are a 500-to-1 to 1000-to-1 density differencebetween the powder and gas and the low flame speed. The faster thestream speed, the easier to keep the powder suspended, but increasingthe stream speed high enough above the flame speed blows it out.

Referring to FIG. 1A, a particle small enough to be explosible has alarge surface-to-volume ratio and burns in a different modality fromnon-explosible larger particles of the same material with a far lowersurface-to-volume ratio. This modality is called single phasecombustion. Yarin and Hetsroni (Combustion of Two-Phase Reactive Media)describe combustion of explosible particles in an explosible mixture ashaving no gas-phase mixing and no gas-phase combustion. For particles inan explosible mixture, the particle temperature does not vary withradius, but only with time. “Single temperature” behavior is part ofwhat makes our combustion process appear essentially single-phase. Forparticles small enough to be explosible, burning occurs at the surfaceof the particle. Combustion time, including time for moistureelimination and devolatization, is short compared to the deflagratingflame front transit time. This type of “flash burn” is nearinstantaneous, but it can be delayed if the combustion becomesdiffusion-limited by the ability of oxygen in nearby air to furthersupport combustion due to lack of supply.

In what is called a heterogeneous reaction, the solid fuel burns in airin a single phase process as if it were a gas or vapor fuel for thecombustion of substantially explosible powder particles, even thoughinitially these two reactants exist in different solid and gas phases.The reaction between the oxygen and the explosible solid particle at itssurface consumes the particle in a flash burn in a single-phase surfacereaction. For particles larger than the explosible limit, the reactionprocess is termed homogeneous, since substantial devolatilizationproduces outgassing of fuel vapor, which reacts with oxygen in the airin the gas phase of a two-phase reaction. In the present disclosure, theterm homogeneous, unless otherwise stated, has nothing to do with theuniformity of the mixture and its lack of gradients in particulatedispersion and temperature.

Particles burning in the explosible powder fuel regime do not outgasvolatiles. As discussed by Yarin and Hetsroni, as the fuel particle sizegoes down, the particle heating rate goes up and combustion time goesdown. At a sufficiently small particle size, pyrolysis time (required tooutgas volatiles), although it has become extremely short, is stilllarge compared to particle combustion time. That is where the processdividing line is. Below that size, oxidation processes take place at theparticle surface at a rate limited by oxygen diffusion, and hence nosensible volatiles are produced.

R. I. Nigmatulin (Dynamics of Multiphase Media: Volume 1) discusses thepropagation of a combustion wave in two-phase processes, which isdescribed as “gas-solid combustible particles”. This process is definedby interactions amongst hydrodynamic, thermophysical, and chemicalprocesses. His theoretical analysis of the interactions of thesephenomena involves a system of equations that treat hydrodynamics, heatand mass exchange, and chemical kinetics in a two-phase medium, withdescriptions of assumptions that can be made to reduce it to singlephase.

The actual mechanism of combustion propagation in mixtures ofcombustible particles in a gas depends on the particle-burning regime,powder fuel concentration, burner combustion chamber geometry, and theactual method of initiation (sustained ignition). Like our process,Nigmatulin states that propagation velocity or flame speed of thecombustion wave front ranges from centimeters to a few meters persecond. We estimate the flame speed response surface range of explosiblepowders tested to be in the neighborhood of half to about a meter persecond, varying with the equivalence ratio.

The three possible particle combustion regimes are: heterogeneous—ourmodality, quasi-homogeneous—the combustion of fuel vapor andgasification by-products, and vapor-phase—the combustion of volatilecomponents.

Particle size is the prime criterion determining the regime ofcombustion. With heterogeneous combustion in a general form, actualburning occurs at both the surface and within the combustible fuelparticle. Heat from the combustion chemical reaction is transferreddirectly to the particles to maintain ignition temperature. Particles ofsmall enough diameter, which are free of or have minimal volatileorganics, such as graphite, electrode coal, and other powders, burn thisway.

Larger particles falling outside the explosible range, with considerablylower surface to volume ratios, combust in a combination of theindicated regimes. Part of the reaction occurs in one regime, in thiscase a vapor-phase regime, followed by another portion of the reactionin the heterogeneous regime.

As particle sizes increase beyond explosible, the time required fordevolatization in the vapor-phase becomes significant. The effect oflevels of percent volatiles becomes limiting. Combustion occurs in avirtual thin layer around a droplet or particle in the vapor-phase inwhat Nigmatulin calls a microflame or an F-phase. Local temperatures arenot uniform, as the spherical flame layer temperature is greater thanthe drop or particle temperature, which in turn is greater than thetemperature of the surrounding gas. Particles of coal, powder,explosives, wood particles and chips in suspension burners, metal anddiesel fuel droplets combust in this regime until their volatilecomponents totally burn out.

Ignition occurs when the particle's surface temperature rises to acertain level. A heterogeneous regime stage of “slow” burning occurs,and if the regime is vapor-phase, the vaporization or gasification stageoccurs. Before any phase transitions occur, the gas heat flux to theparticle surface actually penetrates the particles. In our case, themajor portion of the temperature difference occurs in the gas, so theaverage temperature of the surrounding gas and the particles just priorto ignition are approximately equal, or single temperature. The Nusseltnumber, a dimensionless ratio describes the heat exchange.

For the more specific regime of heterogeneous particle combustion,Nigmatulin states that there is no F-phase in the regime, along with nofuel vapor, and that the heat of chemical reaction is transferreddirectly to the particles. The reaction rate constant for thisheterogeneous particle combustion regime is given by the Arrhenius lawand described by the empirical formula for the dimensionless Nusseltnumber. For our disclosure, the Sherwood number describing the diffusioninflux of the oxidizer to the particle's surface indicates a diffusionregime of burning, not a kinetic one, since at the wave front of thereaction zone, the process goes from kinetic to diffusion, per Yarin andHetsroni.

A candle flame burns in a single phase combustion process, yet it iscommon to get smoke and soot from it. Similarly, a kerosene flame is asingle phase combustion process, yet it too produces smoke, some sootand detectable aromatic vapors. Portable kerosene heaters such as theKerosun® or salamander type heaters prove the point. Most everyone isfamiliar with the smoke produced when two phase combustion processes are“fuel-rich”, running on too little air for a given quantity of fuel.Yard equipment such as lawnmowers, chain saws, snow blowers, and thelike smoke profusely when partially choked and running fuel-rich. Thissmoke signifies that the engine is operating above the properstoichiometric fuel-to-air ratio (FAR). Likewise, fresh logs, thrownatop a hot bed of coals, smolder and smoke prior to eventually burstinginto flames. This phase of combustion is actually producing volatile,rich, combustible woodgas in an operating environment of too little air,more specifically at about three times the stoichiometric FAR. It is bythis basic oxygen-starved process that woodgas is produced for use as analternative fuel.

Unlike with the lawn equipment and stove wood examples above, thepresent inventors have run 80-mesh pine explosible powder (˜177 micronsmode particle size diameter) with so little primary air and so much fuelthat the particles begin to drop out, yet no smoke or soot is produced!This is an amazing phenomenon for a material which only differs from apine log physically in its size. Yarin and Hetsroni describe astoichiometric state of fuel:air ratio as having a value of λ(lambda)=1. This dimensionless number is called the equivalence ratio inthe present disclosure. A rate twice stoichiometric has a λ=2. Ourburner tests, running purposely fuel rich, have had values of λ=3 to 5+,yet burned without smoke or soot from a grossly over-rich fuel mixture.

Although it is a bit of an oversimplification, it is useful to view theissues of smoke, soot, and char production as practically non-existentwhen burning explosible powder in an explosible mixture, whether nearstoichiometric or not. The particles either burn completely and nearlyinstantly if adequate oxygen is present, or they do not burn at all withinsufficient oxygen, simply heating to combustion temperatures. The onlycharred particles collected so far have been significantly oversized andtherefore non-explosible by definition.

The burners of this disclosure take advantage of these facts,particularly the wide range of equivalence ratio that supportscombustion from slightly less than λ=1 (lean) to nearly λ=10 (10×stoichiometric). Simply put, we preferably utilize a two-stagecombustion process as depicted in FIG. 4. The first stage comprises theheating side where the cold mix of air and particle reactants ispre-heated by the gas, and initial combustion begins at the stationarydeflagrating wave. As particles move into the reaction zone, heatingcontinues, now from particles to gas, and combustion continues untilavailable oxygen is depleted. The particle combustion time constant inthis stage is a function of its radius, T_(1st Stage)=ƒ(r).

The second stage begins in the reaction zone when active high speedsecondary air travelling, for example, at 10 times the flame speedenters the burner enclosure, to increase the turbulence and supply muchneeded oxygen to the waiting hot particle mix. Combustion resumes andmoves toward completion of char burnout. Particle combustion in thissecond stage far more rapid, as the time constant is now a function ofits radius squared, T_(2nd Stage)=ƒ(r²).

Far outside of the stoichiometric region, a powder-air mixture of thepresent invention may still be explosible, and yet follow the same lawsas a single phase methane hydrogen gas mixture in air. The surprisingdifference is as follows. A powder-air mixture, acting as a single-phasemixture in combustion, is still explosible at several timesstoichiometric, whereas a true single phase methane-hydrogen-airmixture, with the same over rich λ equivalence ratio (high fuel to airratio) is non-explosible. Surprising but true.

If, for a given low air flow and overly rich fuel mixture, we continueto add powder, further increasing λ, eventually at some point the massis so great and the particles so closely spaced that the feed streambecomes a “thermal ballast”, absorbing the heat of combustion, andburning is not sustained.

When we blow an explosible powder-air mixture into one of our“stovepipe” burners at λ=2 or 3, we get a complete combustion ofportions of the explosible powder mass until the oxygen inside thecombustion chamber is depleted. Initial first stage burning in thewavefront occurs at the surface of the particles and, in this case,becomes diffusion-limited by the supply of oxygen in nearby air tofurther support combustion. At this oxygen-starved operating point, aportion of the powder fuel mass not only remains unburned, but also muchis uncharred with no significant pyrolysis begun. Combustion so far hasremained single phase. The unburned portion of the explosible powdermass continues ballistic travel beyond the dying flame front toward thetop exit of the burner. As will be discussed in detail later, theaddition of secondary air to this oxygen-depleted region demarks asecond stage, where diffusion-limited combustion resumes far faster.

Without secondary air, when the heated, oxygen-starved mixture emergesat the burner top exit, it ignites and combustion recommences. At notime in this oxygen-starved situation is any observable or sensiblesmoke or soot produced. The powdered fuel primarily either burns or itdoes not. We have observed no significant middle ground with particlesizes within the explosible range.

At values of λ below 1, an explosible powder-air mixture follows singlephase gas laws quite nicely, whereas at values of λ>>1, these richmixtures may reach a point where they do not. If λ=2, burning at theparticle surface doesn't change, only the concentration per cubic meterdoes. For explosible particles, surface burning characteristics do notchange and do not matter for combustion.

When an explosible mixture of an explosible powder is blown horizontallyinto an open atmosphere with no containment, the burning may bedescribed as a heat transfer process, where particles arriving at thestationary combustion wavefront are warmed up through gas conduction bythe proximity of others burning. The turbulence that does occur isusually generated by shear around the outer volume of the movingair-powder dispersion in response to the Bernoulli Effect and gasthermal expansion from combustion.

When dealing with small particles of an explosible powder with aparticle size in the explosible range, the percentage of volatiles hasno significant or limiting effect on the combustion process. Whilecombustion articles comparing pulverized coal powders do not exactlyagree with this statement, the conflicts may be related to energyrelease per unit of coal for types with different percentages ofvolatiles, rather than disagreement on the regime of burning. Forsingle-phase and multi-phase combustion, when a particle is small enoughto be explosible, its heterogeneous surface burning behavior enablessingle-phase fluid mechanics approximations to be applied, and indeflagration it behaves as if it were a single phase combustion process.

At most of the desirable operating conditions, a combustion process ofthe present invention is simple, since it is profoundly single-phase inbehavior. This sole, single-phase burning appearance as a solid combustslike propane, renders moot objections, concerns, and challenges that maybe raised about the discovery and uniqueness of our disclosures. Perhapsthe most beneficial result of this regime of burner operation isapproximate 100% combustion completeness with incredibly low levels ofbyproducts (see FIG. 7).

As described by Yarin and Hetsroni, large particles have a very distinctdevolatization state, which requires a longer time to complete than thenear-instantaneous flash burn of particles comprising an explosiblepowder. For particles above the explosibility threshold (200+ micronsfor wood) to the 1-mm range (1000+ microns), devolatization time isrelevant and increasing. For particles too big to be explosible,combustion occurs in a two phase process, involving moistureelimination, devolatization/pyrolysis, outgassing of combustible vapors,and smoke generation, followed by burning of char to essentiallycomplete the combustion.

When operating in the explosible particle size distribution region tocombust substantially explosible powder, only single-phase phenomena areobserved. Pyrolysis-devolatization is not relevant in this region, asthe predominant particle temperature relationship is to time, becausethe temperature gradient across the diameter of the particle this smallis essentially zero.

Multi-phase combustion phenomena are observable when chunks, eitherlarge particles or agglomerates of small ones, are burning. These chunksfollow ballistic trajectories often taking them out of the burner ontothe floor beyond the left-most area of gravitational powder particlesettling shown in FIG. 11.

When particles are larger than the explosible range, the combustionprocess is multi-phase. When particles are within the explosible range,phenomena such as swelling, devolatization and outgassing are notobserved, possibly occurring within milliseconds and therefore not ofconcern.

Time-consuming devolatization is only a concern when the temperature ofa particle varies significantly with its radius, or along the radius(see FIGS. 2A and 2B). This is a typical characteristic of largerparticles outside the explosible range, from coarse powder to sawdust tologs, all of which combust in a two-phase regimen. Devolatization occursand limits combustion speed with significance only outside theexplosible range where we do not claim substantial operation (see FIG.2B).

The flame speed and particularly the primary air-fuel mixture feed speeddifferences from pulverized coal are important to understand. Powderedsolid fuel of the present invention is transported through a nozzletogether with primary air in a dispersion at a low speed above the flamespeed, for example twice the flame speed. The stream slows, diverges,and is ignited, forming a stationary standing wave flame front riding onand burning around the fuel-rich core of the flowing explosiblepowder-air stream.

With a burner of the present invention, there is no requirement that thecombustion zone and enclosure ever be at radiant temperatures like withtraditional and custom methods for burning various forms and particlesizes of pulverized coal. The fact that the stability of ourdeflagrating flame front is well-maintained and supported in aheterogeneous combustion mode without requiring any radiant heattransfer is a key point of uniqueness, differentiating our inventionfrom prior art.

As a further example and comparison, classic techniques include feedingpulverized coal mixed with primary air in a suspension into afurnace/burner at a rate of twenty or more times the flame speed, aprocess parameter that would not work within our invention disclosure.Only by virtue of sending the coal premix into a reactor with controlledhigh temperature radiating walls is sustained and complete combustionpossible under these conditions. For our invention, burner walltemperatures are not important to initiate and sustain combustion, andno large ideally-stirred reactor is required. These are significantdifferences.

For direct comparison, in the prior art, dry pulverized coal may betransported through a nozzle together with primary air at a high speed(up to at least 56 ft/s, according to Y. Kwan et al. in “AdvancedCoal-fueled Combustor for Residential Space Heating Applications”), morethan an order of magnitude above the flame speed (10 to 30 timesgreater) into a burner, which is a small reaction chamber emulating alarge one. Some designs then continue the combustible flow into theactual furnace, a large reaction chamber where combustion is completed,with ignition initiated by the mechanism of radiation heat transfer (P.M. Krishenik, “Modeling of Combustion Wave Propagation in a CarbonDust/Gas Mixture”), rather than particle-to-gas heat transfer as isutilized in the case of our combustion burners.

Ballester et al. tested a prototype burner with two types of coal,bituminous and lignite, and oak sawdust powder. The design was to burnpowder in a regime where heat transfer was primarily radiant as in ahigh temperature reactor. The large power station-oriented coalcombustion experts, like all their peers over the years, completelymissed the fact that their two coals were pulverized in the 20 to 40micron range and were in fact of explosible size. Rather, they performedtheir tests and comparisons with biomass oak dust having particles toolarge to be explosible (χ_(w)<<1), by blowing the fuel into a hightemperature chamber of 2000° F. or higher, often at 40 to 60 times theflame speed. In such chambers, the vast majority of the heat transfer isradiant from the chamber walls, thereby igniting and combusting theentire distribution in a two-phase regime, where time and space are notsignificant limitations, and the value of χ_(w) does not matter much.

It is important to remember that in most large furnaces there exists aniron-clad lower limit on combustion rate, the minimum fuel mass flowrate to keep the combustion zone at combustion temperatures so thatignition of particles, large and small, continues as new fuel reactantarrives. The net result is that furnaces and burners designed to emulatetheir high temperature radiation-based ignition environment have a farnarrower operating range and a far smaller turndown ratio than a farsmaller burner of the present invention.

It is also important to compare to pulverized and ultrafine coal powdercombustion burner strategies utilized in this DOE final report from 1989by Y. Kwan et al. Our whole concept is different than the strategiesutilized in pulverized coal powder combustion burners and large powerplants, as demonstrated by the following combustor design andoperational comparisons:

A burner of the present invention is not required to be integrated witha big furnace-radiating reactor volume for combustion completion. Noatomization is required. Our primary air-fuel mixture is fed from thenozzle at just above the flame speed, whereas a coal powder premix issprayed at 10 to 30 times the flame speed. Our burner is completelystandalone for operation, while coal burners must be connected to alarge furnace refractory. Our burner is small in size, not burdened bycoal combustion design goals and complexity common to large furnaces.

The DOE coal combustor required pre-heating the fuel, a quarl andrefractory (a 10% energy loss), pre-heating of secondary air, as well ashot gas injection and/or external flue gas recirculation. We have noneof these requirements!

Our burner preferably uses a simple spark igniter and continues withself-sustained combustion and no further or external ignitionintervention required. The DOE combustor needed oil or gas fuels for apilot light to initiate and initially sustain ignition, heat thecombustor, and stabilize the flame. We require none of those heatingfunctions or hardware. While our burner is self-sustaining through asimple conduction regime, the DOE unit design emulated the radiationheating found in typical coal furnaces to ignite particles continuously.No standard coal combustor would sustain a flame if disconnected fromthe irradiative furnace reactor! Our burner is standalone. When it comesto size, the DOE burner's “BTUs per hour per cubic foot” is farlower—perhaps one-tenth of the output per cubic foot of a burner of thepresent invention.

However surprising, our burner can in fact burn cold, dry powdered coalof the proper particle size using our fluid mechanics combustion regimejust described.

When secondary air is included, as in a number of embodiments of thepresent invention, combustion of an explosible fuel-air mixture beginsinstantly in an initially cold burner, is sustained through kineticparticle to gas conductive heat transfer, and is completed substantiallywithin the burner itself, rather than in a large, pre-heated furnacereactor driven by radiation heat transfer, which is typical art forpulverized coal.

The DOE-sponsored final report on combustion devices for home heatingusing coal included pulverized coal industry combustion design andoperating principles, which are clearly different from burners of thepresent invention. Their burner took 70 minutes to come up to operatingtemperature, even using pulverized and dry ultrafine coal (DUF), whereasour burner has practically instant ON-OFF capabilities and can berunning in the operating region in well under 5 seconds.

A preferred use of our burner in many applications is as a singleburner. It is not initially intended to be used in vertical banks orarrays of multiple near or below horizontal burners, although there isno intent to be limited by this design goal. It is not intended to burncoal dust slurries. It is designed to operate in a single-phasecombustion regimen.

Others have chosen to pulverize coal prior to combustion for a varietyof process and economic reasons. Coal is far easier to pulverize thanmany biomass and other powder source fuels. Some coal particledistributions have contained a significant amount of particles in theexplosible range, others substantial, yet they have not been combustedusing the theory and practice of the explosible mode and single-phasecombustion regimes we disclose. The same holds true for various types ofhog fuel. Rather, this pulverized fuel has been and is being combustedusing processes and two-phase regimes with classic burner and furnaceideally stirred reactor technology, hardware, techniques, andapproaches.

One skilled in the art may easily differentiate the present inventionfrom the prior art by noticing what is not important or required topractice our disclosure versus current art examples given. Thiscomparison, combined with the radically different fluid mechanic singlephase regime emulation and methods practiced further substantiates theuniqueness of our disclosure.

DETAILED DESCRIPTION OF THE FIGURES

FIG. 1A shows two curves 10, 11, conceptually depicting two particlesize distributions. An important component to our unique powdered fuelenergy conversion process is our preferred use of a substantiallyexplosible powder as a fuel, with particle sizes from a few microns upto the neighborhood of 200 microns, as seen in curve 10.

Wood particles much larger than 200+ micron limit are not typicallyexplosible, burning more slowly in a common two phase regime. The actualsize limit for explosibility varies for different types of biomass andother explosible powders based on a number of variables which include,but are not limited to, particle surface area-to-volume ratios, particleaspect ratio, percent moisture, percent volatiles, calorific value ofthe powder/dust, temperature, dispersion concentration, particleinternal structure morphology, and the like. The term 200+/− microns isused to denote the dividing line between explosible and non-explosiblepowders generically in this disclosure. The actual particle sizediameter for different types of biomass and other powder fuels may befound in a variety of resources (Dust Explosions in the ProcessIndustries, R. K. Eckhoff).

In some ways, a basic measure of the explosibility of a particledistribution is the ability of that distribution, including its largestparticles, to flash burn in an explosible mode in the various burners wedisclose in tens of milliseconds, emulating a single phase combustionregime commonly seen with propane and other liquid and gaseous fuels.

The distribution 11 on the right of FIG. 1A includes a wide range ofparticle sizes, with a predominant membership in the non-explosiblerange. Wood chips, saw dust, ground waste, hog fuel, coal, and othercombustible biomass up to whole trees and hydrocarbon-based fuels havebeen burned in large furnaces for boilers, power plants, and othercommon modes for years as cited in numerous references in thisdisclosure. More recently the literature describes mixed fuel andco-fired burners and combustion schemes for predominantly non-explosibledusts and powders. While a portion of the particle size distribution mayfall into the explosible range, today's technology applications tend toproduce energy with large mean particle sizes by comparison, often bystated choice to avoid the explosion dangers of particle dust and fines,which fall into the explosible range. Particle size distribution is amajor differentiator between our disclosure and current art.

FIG. 1B depicts an ideal particle size distribution 12 centered aroundthe 50 to 80 micron mean, and a more typical curve 13 found in varioustypes of substantially explosible fuels from biomass and other powderedsources. This curve 13 is skewed heavily to the right, toward a modelarger particles than the mean or median would indicate, yet is stillwithin the explosible region. The skew is primarily based on themanufacturing processes, from sieving to more selective separationtechniques utilized.

As with all manufacturing processes, there tends statistically to be aminor portion of the overall distribution which may fall just outsidethe desired region. This amount is a somewhat adjustable quantitydepending on economic throughput models combined with thereproducibility of the manufacturing and separation equipment. For someuses, control of this right-hand tail of the curve accounts fordifferent quality levels or grades of fuel.

Three different shapes of substantially explosible powder distributions15, 16, 17 are depicted in FIG. 1C. The particle size distributions forembodiments of the inventions herein may have a variety of statisticalcharacteristics, based on uses and economics.

By selecting powder dispersions wherein the powder size distribution hasa median mode represented by curve 16, dispersions are achieved that areexplosible regardless of variables such as the surface area of thepowder particles, the energy content of the powder, the concentration ofan oxidizer such as oxygen, the temperature of the powder and theoxidizer, and the heat transfer rate, provided that sufficient oxidizinggas is present to qualify as explosible in nature. Accordingly,embodiments of inventions herein are capable of deflagrating dispersionsof powdered fuel with little or no adjustment required for variouspowder materials.

FIG. 2A depicts the general relationship 20 between particle size of apowder particle and the particle combustion time. Dashed line 14 depictsthe explosible limit for the powder—a threshold above which thedispersion is not explosible. This threshold varies from dispersion todispersion and the other noted factors above. For example, with respectto the concentration of an oxidizer, a first dispersion including aparticular powder may not be explosible where the dispersion include airhaving 20.95% oxygen, while a second dispersion including the samepowder may be explosible where the dispersion includes pure oxygen. Formethods and apparatus for determining a powder's explosible limit, seeW. Bartknecht, Dust Explosions: Course, Prevention, Protection.

FIG. 2B shows there is a generally inverse relation 22 between the heattransfer rate and the particle size, with the most predominant portionof the curve existing in the explosible powder region. The heat transferrate for smaller particles is generally higher than for largerparticles. Again, dashed line 14 depicts the explosible limit. The curveof FIG. 2B provides the explanation for why fuels composed primarily oflarge particles on the order of 500 μm must remain in a furnace for asignificant period of time.

FIG. 3 shows a matched set of graphs taken from Eckhoff, the firstdepicting the minimum required ignition energy 30 as a function of fuelconcentration for an explosible powdered fuel dispersed in an oxidizinggas. An explosible powder is only explosible in a concentration rangewith an oxidizing gas between a lower concentration limit 32 and anupper concentration limit 34. Above the upper critical concentration,the concentration of oxidizing gas is too low to burn all of the fuel.Below the lower critical concentration, the particles are too far apartfor enough heat to be transferred from burning particles to unburntparticles to ignite the unburnt particles. The minimum ignition energyrequired to ignite the particles has a minimum within the explosibleconcentration range at 36.

FIG. 3 also shows schematically the flame speed 38 as a function of fuelconcentration for an explosible powdered fuel dispersion in an oxidizinggas. Again, flame speed is relevant only in the explosible particleconcentration range between a lower concentration limit 32 and an upperconcentration limit 34. This curve is reminiscent of fluid mechanicdepiction of the flame speed versus the equivalence ratio λ.

From the industrial dust explosion perspective, these curves represent acomparatively narrow explosion range. However, from our combustionprocess point of view, the range of concentrations in which asingle-phase combustion regime operates is rather wide and generous. Theleast energy required to initiate the most violent explosion with thehighest flame speed is at a concentration somewhat greater thanstoichiometric where λ>1. Also noteworthy is that the range ofexplosibility for a dust dispersion of solid particles in relation tostoichiometric concentration is far greater than any ranges found withliquid or gaseous fuels such as propane, methane, or gasoline.

FIG. 4 graphically depicts combustion of a stationary deflagrating flamewave front 40 at the interface of a moving stream 42 of premixed solidparticles in an oxidizer.

In this embodiment, we are feeding a powder from the base of ahorizontal auger, mixing it with air into a dispersion, and then feedingthat powder-air mixture through a nozzle into our burner at aconcentration considerably over stoichiometric, λ=3-4 with a velocityabove the premix flame speed. Combustion occurs essentially as astanding wave front inside the burner, balanced on the slowing andwidening powder dispersion in a zone where its concentration is lessenedand turbulent mixing occurs through recirculation. We call the followingtwo-stage combustion.

In the first stage 45, the primary process operating in Preheat Zone I44 is the heating of the dispersed phase. The flame front is thetransition line into Reaction Zone II 46 where heating of the gas is theprimary dynamic. The diagram emphasizes the continuous gas-particleconductive heat transfer between the Preheat Zone I 44 and the ReactionZone II 46 as a fresh supply of particle reactants are continuously fedinto burners described herein for deflagration. A graphical temperaturereaction profile 48 is overlaid inside the burner.

In the second stage 47, oxygen is depleted somewhere in the reactionZone II, while hot particles at combustion temperature continue moving.The second stage begins with the introduction of high-speed secondaryair 49 at an angle to encourage mixing and a velocity perhaps 10-timesthe flame speed. Adequate oxidizer drives char burnout to completion, afast process that occurs in a time related to the particle radiussquared (R²), rather than just R as in the first stage.

FIGS. 5 and 6 present a basic combustion and fluid mechanic summary ofphenomena and detail the radically different behavior between combustionof explosible versus non-explosible particles. A list of comparisonterms or phrases is overlaid onto the shape of the explosible andnon-explosible particle size distribution curves of FIG. 1A to emphasizethe unique differences between our operation and prior art.

FIG. 5 summarizes basic combustion phenomena observed when our discloseddevices and fuels are operating in the explosible regime compared withoperation in the non-explosible, more traditional combustion mode. Eachitem in this overview summary of key combustion regime differences isdiscussed in detail in other portions of our disclosure. The summary,taken as a whole, makes clear the novel and surprising nature of ourdisclosure in combustion terms familiar to many including those skilledin the art.

FIG. 6 summarizes fluid mechanical processes and phenomena observed whenour disclosed devices and fuels are operating in the explosible regimecompared with operation in the non-explosible combustion mode broadlyutilized today. Each item in this overview summary of key combustionregime differences is discussed in detail in other portions of ourdisclosure. The summary, taken as a whole, makes clear the novel andsurprising nature of this unpracticed combustion regime described in ourdisclosure using fluid mechanics and combustion dynamics terms familiarto those skilled in the art.

A burner system of the present invention preferably includes fivesections. FIG. 8 shows schematically a burner in a first embodiment ofthe present invention. The burner system 80 includes a powdered fuelfeed system 82 for feeding a powdered fuel and an oxidizing gas feedsystem 84 for feeding an oxidizing gas. The powdered fuel and theoxidizing gas mix in a mixing zone 85 fed by the powdered fuel feedsystem 82 and the oxidizing gas feed system 84. An explosible dispersiondirecting system 86 fed by the mixing zone 85 directs the explosibledispersion toward the ignition source 88 in the confines of an enclosure(not shown). The ignition source 88 is located downstream from theexplosible dispersion directing system and initiates a deflagratingflame for the explosible dispersion. The powdered fuel feed system 82,the oxidizing gas feed system 84, and the mixing zone 85 arecollectively termed the positive displacement powder dispersion feedsystem. Each of these parts is described in detail below.

FIG. 9 shows schematically a burner system in a preferred embodiment ofthe present invention. The powdered fuel feed system 82 includes apowdered fuel storage container 91, a fuel vibrating device 92, a fuelmetering device 93, a usage meter 94, and a fuel feed power source 95.The vibrating device 92 vibrates the fuel storage container to reduceclumping of the fuel and to maintain flow of fuel, preferably bygravity, from the storage container 91 to the metering device 93. Thefuel supply power source 95 controls the rate of fuel feed bycontrolling the metering device 93, and the usage meter 94 records theamount of fuel fed the metering device 93. The oxidizing gas feed system84 includes an oxidizing gas source 96 and a gas metering device 97. Theexplosible dispersion directing system 86 includes a constriction device98 and a flame stabilizing system 99 downstream from the nozzle. Anagglomerization device 100A, 100B, 100C to break up fuel agglomeratesthat may have formed during transport or storage of the powdered fuelmay be optionally located within or after the fuel metering device 93,within the mixing zone 85, or within the flame stabilizing system 99.

A burner system of the present invention is preferably turned on byinitiating the powdered fuel feed system to provide powdered fuel at apredetermined feed rate and actuating the ignition source to ignite thefuel and produce a deflagrating flame. The oxidizing gas feed system maybe initiated before, at the same time, or after the powdered fuel feedsystem is initiated, and the oxidizing gas from the oxidizing gas feedsystem carries the powdered fuel to the combustion enclosure past theignition source.

A burner of the present invention is preferably sustained in the onposition by continuing to feed powdered fuel and oxidizing gas atpredetermined feed rates, which may vary with time depending on loadrequirements, to the deflagrating flame.

A burner of the present invention is preferably turned off by turningoff the powdered fuel feed system. The oxidizing gas feed system ispreferably turned off at the same time as the powdered fuel feed systembut may be turned off before or after the powdered fuel feed system isturned off. Alternatively, the oxidizing gas feed system may bemaintained at a predetermined flow rate, which may vary with time, whenthe burner is both in the on and off states.

Powdered Fuel Feed System

The powdered fuel feed system preferably includes a powdered fuelstorage container, a fuel vibrating device, a fuel metering device, ausage meter, and a fuel feed power source. The powdered fuel storagecontainer may be any size or shape, preferably with a downward slopingbottom, and made from any structural material. The container ispreferably easily accessed for addition of powdered fuel to the systemas necessary or fed from remote storage. The fuel vibrating device maybe any high frequency device which promotes flow of the powdered fuelfrom the storage container to the fuel metering device and reducesagglomeration. The fuel metering device may be any device capable offeeding a solid material at an adjustable and controllable rateincluding, but not limited to, a screw auger, a conveyor, a rotary diskor other metering devices. The usage meter may be any device that countsand records the usage of the fuel metering device, such as the number ofturns of a screw auger, to determine the amount of fuel being used bythe burner system. The fuel feed power source may be any power source,but is preferably electrical, and either separate or the same powersource may be used to run the fuel vibrating device and to drive andcontrol the rate of the fuel metering device.

Oxidizing Gas Feed System

The oxidizing gas system preferably includes an oxidizing gas and a gasmetering device. The oxidizing gas for the oxidizing gas feed system maybe air, oxygen, or any other composition of gas containing oxygen. Theoxidizing gas source may be ambient air, compressed air, or compressedoxygen. The gas metering device may be a valve, a pump, blower or anyother device to control the feed rate of the oxidizing gas. The powersource to run the oxidizing gas feed system and the power source to runthe powdered fuel feed system may be the same or different powersources. Oxidizing gas is separately fed to the burner as a source ofsecondary air to support one or two stage combustion.

Mixing Zone

The mixing zone is a zone of the burner system which allows theoxidizing gas and the powdered fuel to intersperse after they cometogether. The mixing zone ideally allows the oxidizing gas to break upand distribute the powdered fuel into its individual particles so thatit behaves as a reasonably uniform explosible powder when it reaches thedeflagrating flame. The mixing zone may be a chamber, a conduit,educator, or a combination of chambers and conduit. The mixing zone ispreferably designed to produce a dispersion by turbulent flow of theoxidizing gas.

Ignition Source

An ignition source for a burner of the present invention is used to turnthe burner on by initiating a deflagrating flame in the fuel-gasdispersion fed by the PDPD system and formed by the flame stabilizingsystem. Since a burner of the present system preferably stays on untilthe fuel supply is cut off, the ignition source may be pulsed ON/OFF orcontinuous. The ignition source is preferably an electric arc ignitionsource or other spark source such as a conventional spark igniter. Theignition source may, however, alternatively be a gas flame pilot light,a glow plug, or any electronic igniting device.

Explosible Dispersion Directing System

The burner, or explosible dispersion directing system, receives themixed dispersion from the PDPD through the nozzle and delivers it to theignition source for initiation of the deflagrating wave. The explosibledispersion directing system also controls the process by takingadvantage of the fluid mechanics of the dispersion and the deflagratingwave to optimize the burner for the specific application for the burner.The explosible dispersion directing system includes a constrictingdevice and a flame stabilizing system. The constricting device controlsthe speed and area of the explosible dispersion as it is fed into theflame stabilizing system. In one embodiment, the constriction device isa nozzle. The flame stabilizing system is designed to control combustionmakeup air system of the powdered fuel and may include an active or apassive secondary air system as well as a de-agglomerization system.

A number of tests were run burning horizontal, uncontained air-powderdispersions to learn about the dynamics of combustion before actualburner design and testing began. A short series of tests, using a leafblower to supply air to the burner, were instructive. We learned thefollowing: 1) Flame speed is important and relatively low for explosiblepowders. Full power through a 3-port dispersion nozzle touting 200 mphair velocity is just plain too much. It was nearly impossible to ignitethe powder-saturated air stream closer than 4 feet from the end of thenozzle.

2) Lowering the air stream velocity (and flow) allows ignition to occurcloser and closer to the exit nozzle. With these rather high velocities,the non-ignited air-fuel mix was still traveling faster than the flamespeed and therefore the burning zone itself.

3) The lowest motor speed for both velocity and flow produced anexplosible mix that is reminiscent of a flame thrower. Only at thelowest leaf blower velocities did the flame spread laterally from theignition point.

4) The requirement for and merits of a robust, consistent meteringsystem to feed a fuel/air pickup, mixing system, and delivery system washighly apparent, as this crude but instructive demo didn't really haveone.

5) The initial flame front formed only if the mass flow velocity of theair-powder mix falls somewhat near or slightly above the deflagratingwave front velocity. Then burning move back (upstream) and stabilizefrom the ignition source. When this happens, a self-sustaining burn ispossible after ignition with containment.

FIG. 10A shows schematically a horizontal explosible powder fueldispersion in an embodiment of the present invention with no flame. Thepowder fuel dispersion 100 depicts the fluid dynamical behavior. Flowexiting the eXair air amplifier in much of the cone is initially adirected stream. Turbulence quickly begins circumferentially at the edgeof the dispersion flow as stationary air is induced, producingnoticeable eddies and drag vortices. The high velocity dispersion jetflow produces a high shear, resulting in turbulence as stationary airflow is induced by the fast moving air-particle mixture. Eddies andwaves 104 are immediately apparent surrounding this diverging cone andcontribute to a slowing of the air-particle mass flow even beforegravity effects become pronounced.

Velocities are high near the center of the slowly diverging cone and notimmediately amenable to ignition or to sustained burn. There is littleor no visible evidence of any particle fall-out in the first severalfeet after the amplifier nozzle exit. The fine unignited powder, whichis easily seen without ignition, tends to disperse evenly throughout theroom, not just fall to the ground as discharge velocity approaches zero.Particles remaining in a slowly-diverging cloud form a uniformdispersion of fine powder.

FIG. 11 depicts the effects of gravity and turbulence on the velocityand momentum of an unignited horizontal dispersion 110 suspended solidsblown into stationary air. The first region is characterized by a highvelocity flow surrounded by high shear, low pressure room atmosphere andresulting edge flow drag induced turbulence and consequent mixing aspredicted by Bernoulli. These phenomena continue for about four feet,abating into the second region of slowing velocities and reducedmomentum, where the dispersion appears highly uniform except for someturbulence and thinning at the extreme edges.

At about eight feet the dispersed particles enter the third region wherethe dust-like powder begins to settle 112 under the influence of gravitydue to a loss of momentum. Any undesirable oversized, higher massparticles, especially ones within the specified diameter but with aspectratios greater than 2, stay in a ballistic mode until the end of thethird region where they too succumb to gravity.

FIGS. 12A through 12D show unconfined free space ignition of ahorizontal dispersion as the ignition point location is successivelymoved away from the nozzle source at four increments. Much insight intothe deflagration of a substantially explosible air-powder mix can begleaned from detailed observations and image analysis of the variousphysics and fluid mechanic phenomena at work in this test.

The air-powder premix enters the eXair air amplifier where finaldispersion and velocity amplification are completed. The explosiblepowder dispersion exits the eXair nozzle initially in a fast moving,slowly diverging flow at a concentration several times stoichiometricwith a velocity well above the dispersion flame speed.

At our preferred process feed settings, a propane torch applied directlyat the nozzle outlet does not ignite the mixture, since the velocity isstill above the flame speed, which ranges from about 0.5 to 1+meters/second. The high solid fuel concentration is likely in theexplosible range, but ignition is velocity inhibited.

Within about two inches from the nozzle, a deflagrating flame erupts atthe ignition point 122 of FIG. 12A as shown in the top flame profile.With combustion supported by a propane torch, stationary room air isentrained by the fluid mass flow, with turbulent eddys moving forwardwhile slowly rotating back towards the source as a result of shear. Thisalters the air-fuel mixture around the circumference in the cross flowdirection. The “endless” supply of room air entrained by the highvelocity stream is also a source of combustion air, following theBernoulli Principle. “Slower dust” combusts better as it is near or atthe flame speed.

The flame is not self-sustaining in this embodiment, but will continueto burn in a stable mode as long as the ignition source propane torch isheld vertically in place. Variations observed are due to turbulenceinterplay with the constant expansion and upward flow of hot gas as allof the solid fuel is consumed. This solid fuel stream literally “burnslike propane” with no observable or sensible odor, soot, or remainingparticles.

At the ignition point 122 of FIG. 12A there is no upstream nordownstream movement of the deflagrating flame front wave, as there is amatch of dispersion mass flow velocity with the substantially explosibleair-powder mixture's flame speed. The flame front line is tiltedslightly leftward but nearly vertical across the horizontal flow stream.

As the igniter is moved leftward (downstream) to the ignition point 124of FIG. 12B about 8 inches from the nozzle exit, the deflagrating flamecontinues to consume all of the dispersed solid fuel powder suspensionin an explosible mode. As velocities and concentration have decreasedsomewhat, the actual flame front moves upstream 2 to 3 inches from theignition point and remains essentially stationary. A typical burn at theproper ignition point delivers near complete combustion, with little orno particle fallout.

The ignition point 126 of FIG. 12C is just over a foot downstream fromthe nozzle exit. A noticeable backfire is present, reaching about 4+inches back towards the source, where it remains stationary as a streamof fresh reactants arrives. The far left “tail” of the flame wavers morewith ignition this far out, as the effects of turbulent mixing, thereduction and resupply of oxidizer, and upward gas expansion fromparticle burning have increased effects on the variability of dispersionuniformity and therefore combustion.

The ignition point 128 of FIG. 12D is located about 16 inchesdownstream, the farthest from the nozzle source in this series of tests.A noticeable change in burning is observed, as the dispersion has becomeincreasingly disturbed in its travels, widening both geometrically aswell as in stoichiometric values of air-fuel concentrations. Unburnedpowder is observed surrounding the deflagrating flame cloud, atconcentrations below explosible or temperatures that combine to precludeits combustion consumption. Ignition point 128 is considered to beoutside the desirable operation region of our preferred methods andprocesses, but is included here to round out our disclosure.

During these tests, we also observed the usefulness of the eXair airamplifier with horizontal dispersion. Specifically, when operatingwithout an ultrasonic deagglomeration function, the air amplifierturbulence performs a final destructive breakup and mixing of anyremaining particle agglomerate and thereby reduces or eliminates anyresulting particulate “fall out” around the burning stream, whencompared to a simple pressure-driven stream from the mixing zone to thenozzle.

Introducing a typical continuous spark fuel oil igniter electrodestructure into this fluid stream causes local turbulence andgravity-induced fallout of particles about 8″ to a foot downstream ofthe igniter location, never to be ignited. This simple test made itclear that no structures may be located within the gas-powder dispersionfuel flow, as collisions with stationary surfaces interfere with properand desired operation.

At low powdered fuel and oxidizer flow (about a 10:1 turndown ratio),these same basic phenomena are observed, yet the scale is shrunkconsiderably. The length, diameter, and volume of the deflagrating flameare significantly and proportionately reduced, as are the ignition pointdistances from the nozzle. In home heating terms, initial tests were ata typical furnace heating rate of about 200,000 BTU per hour, with lowflow emulating a hot water heater demand at about 20,000 BTU per hour.

Vertical firing in FIG. 13 and beyond has some advantages. It is rathercommon with gaseous and some liquid burners, but not with solid fuels.To achieve symmetrical effects on the powder particulate dispersionbefore and during combustion, investigations were begun with a verticalair-fuel mixture flow utilizing principles of physics, dynamics, andfluid mechanics to create simple upward firing vertical combustionenclosures for substantially explosible powder dispersions. A verticaljet, rising up from an infinitely flat plate, follows certain flowpatterns, which allow powder to drop out without burning. In our case,we have an immense density difference between powder and gas, somewherebetween 500:1 and 1000:1.

The burner on a gas stove does not need to be confined in order to workwell, over a very wide dynamic range. It sends a premixed,close-to-stoichiometric, preheated, well-distributed mix out aflameholder, where it burns cleanly. It helps to have an effectivelyzero fuel-air density difference, and truly single-phase operation.

With a horizontal, low-speed jet of explosible powder, keeping thepowder confined in the combustion zone is a challenge. Our horizontaljet, blowing powder out into free space have worked best with powders of200 mesh. When we used fuels comprised of larger particles withdistributions beyond our specification or non-reduced agglomerate suchas 80-mesh pine or baking flour, some of the powder burned, whileanother portion followed a ballistic path and was eventually steered bygravity unburned onto the floor.

A vertical jet, blowing up into free space, had similar, yet less severeproblems, as the effects of gravity are uniformly distributed around thecentral flow axis, eliminating the asymmetric particle behavior.

The purpose of the initial vertical can burners, and the later stovepipemodels, was to symmetrically confine the primary fuel-air flow such thatthe powder would stay in the air dispersion, while utilizing turbulentmixing to promote rapid heat transfer from the gas to the particles,thus maintaining a high enough temperature environment for sustainedcombustion with a stationary deflagration wave front.

A vertical dispersion allows all powder, especially that which is toodense or coarse, to stay suspended and confined in the jet combustionzone and burn in the stovepipe burner. It should be understood that atvery low flow rates, the performance of large burner diameters begins tofall off and destabilize, as the side walls are far enough away to besimilar to “free space”, not providing the necessary turbulentrecirculation.

The following section reviews a number of burner explosible dispersiondirecting system designs developed, fabricated, and tested in somewhatof a chronological order.

FIG. 13 shows a first can burner embodiment of the present invention.The first sustained burn was accomplished when the air-fuel dispersiongenerated by the PDPD and dispersion directing system was fed verticallyinto the first vertical burner 130 enclosure design at a velocity justabove the flame speed. This first burner was fabricated from a 6-inchdiameter×6¾-inch high coffee can. It produced a tall, rather lazy deeporange deflagrating flame rising several feet above the can with nosensible smoke or soot.

It was immediately obvious that further answers would be found buried inthe complex relationship between powder-air mix flow, turbulent mixing,available combustion air, dynamics of a burning gas wavefront, gravityeffect on explosible and larger burning particles, and actual streamvelocity of the air/powder mixture.

From this first sustained combustion burner test, we learned a verticalburner allowed turbulent, recirculating air flow to “bring back” anyheavy burning or unignited particles along with hot gas mixing. A largeamount of turbulence was present inside the can, with a considerabledowndraft around the edges to supply this air-starved situation. Thisdowndraft introduced strong shear and thereby added to the mixingeffects of turbulence on an otherwise uniform flow in a tight boundaryzone formed between the inside walls of the medium sized coffee can andthe periphery of the flow cone.

The nature of this downdraft, or backdraft, returned considerableparticles back toward the source, further fueling and sustainingignition by the main flame. The “combustion zone” ambient temperaturerose dramatically, maintaining sustained ignition/combustion. Much ofthe heated and combusting air/fuel mixture did escape the top edge ofthe coffee can burner. With a simple yet novel combustion chamber, thecombusting dispersion was slower to depart the source. We later learnedthat this was due to fuel particle size imperfections. After the initialignition and rapid subsequent mushroom cloud-like explosion, thecompleteness of the combustion seemed to decrease and the particlenature of the flame and sparks became much more pronounced, as didturbulent air swirling around the main flow cone. However, burning wasself-sustaining.

Both the larger 80-mesh softwood pine and 200-mesh hardwood producedmore sparks with a vertical burner than a horizontal dispersion,especially at the top and plume periphery. These sparks were laterunderstood to be the presence of oversized particles in a less thanperfect explosible dispersion, plus an occasional larger particleagglomerate.

FIG. 14 shows a second can burner 140 of the present invention with foursecondary air holes 142 located near the bottom of the burner exteriorsidewall. The air holes at or near the burner base provide what we calla supply of passive secondary air to assist with combustion via severalprocesses. External air is pulled or induced into the burner enclosureby the force of a negative draft generated in the combustion process anddispersion in-feed. The amount of passive secondary into the burnerthrough these apertures is related to a number of parameters, includingthe pressure drop through the burner, the fuel flow combustion rate, andthe combustion chamber stack height.

The presence of secondary air from the four 1-inch holes on the burnersidewall base improved flame stability inside the burner and allowed fora wider range of air-powder flows without flameout. The structure of theflame above the burner exit was less “lazy” and less tall, as combustionwas completed at a decreased distance from the nozzle.

FIG. 15 shows a smaller can burner 150 of the present invention withsecondary air holes 152. This 4″ diameter can is a scaled down versionof the original 6″ burner. As the nozzle diameter did not decreaseproportionately for this embodiment, less of the combustion occurredwithin the can, appearing more like a torch. A combustion chamber ofthis size, possibly with a decreased infeed nozzle diameter, would beuseful for lesser heat load applications such as a 20,000 BTU/hourresidential hot water heater.

In dealing with a 500:1 to 1000:1 density difference between powder andair and a low flame speed, a goal was substantial combustion completionwithin the burner itself, while containing, supporting, stabilizing, andlevitating this rather high mass suspension for consumption by thestationary “traveling” combustion wave flame.

The fast vertical dispersion jet technique results in a higher initialvelocity and greater shear than the target velocity. It introducesturbulence by the dissipation of kinetic energy in the primary fuel-airmix stream. Higher speed primary flow supports the weight of the powderand allows for a closed off bottom, thereby producing recirculation,sustained ignition, and combustion inside combustor. This design allowsfor a flat bottom, producing an abrupt pressure discontinuity, which isa major driver for recirculation and enables a wide operating range offlows and BTU outputs for a single burner size.

When the primary air-fuel stream enters our burner enclosures at highspeed into the combustion chamber, this reduces the pressure, so airrushes from the surrounding area through the holes into the low pressurearea. This recirculation and turbulence generation in our burners isexplained by the Bernoulli Principle (Bernoulli's Law), which predicts adecrease in pressure in a direction perpendicular to the flow of a gasor fluid. Bernoulli's Equation is a consequence of the conservation ofenergy as applied to an ideal fluid, dictating that the sum of thepressure, the kinetic and potential energies (both per unit volume) is aconstant value at all points along a streamline or laminar flow.

The velocity difference as the air-powder stream exits the nozzle intothe combustion enclosure means there will be a pressure difference,since moving fluids exert pressure on stationary fluids. Recirculationand turbulence are initiated by the resulting energy transfer from thekinetic high speed air-fuel flow entering this enclosed environment.Likewise, the high speed stream induces the flow into the passivesecondary air inlets near the base of many burner embodiments. Onceignition of the air-fuel mix occurs, hot gas expansion adds furtherkinetic energy into these dynamic processes.

FIG. 16 shows two stacked burner cans 160, 162 with secondary air holes164 at the base of the lower can 162. The addition doubled thecombustion chamber length to about 13 inches and reduced the externalflame height by providing an environment for further combustioncompletion. The increased stack height of the combustion chamberincreased the vacuum to induce passive secondary air into the 1-inchholes at the burner base, yet the overall mixture combustion becameoxygen-starved well below the top exit, indicating further secondarycombustion air was needed.

FIG. 17 shows the upper 170 of the two stacked cans 170, 172 movedvertically upward to create a one-inch air gap 174 between the upper andlower cans of the combustion enclosure with the lower can having four1-inch diameter passive secondary air holes 176. This gap 174 provides asecond and additional source of passive secondary combustion air beyondthe four holes or slots at the base. By providing this additional airthrough the gap, the flame height was lowered and more combustion of theoxygen-starved mixture was completed within the now 14-inch tallcombustion enclosure.

FIGS. 18A and 18B show a burner can 180 with a sloped bottom 182 andadjustable bottom air holes 184 as an alternative technique forproviding a more controlled flow rate of passive secondary air with anaxial rather than cross flow orientation. The sloped bottom functionedas intended by returning any unburned particulate back into the airstream. The ability of the bottom holes to control secondary airflowamounts was useful. An apparatus with adjustable slots or holes may besimilarly used on the sides of the combustion chamber enclosure as analternative to this method.

Also confirmed by this embodiment was the value of the cross flow, latercalled the “sweeper function” of passive secondary air, accomplished inprevious prototypes by air inlet holes near the bottom of the enclosure.This cross flow provides greater radial recirculation and increasedturbulent mixing, whereas this more axial secondary air flow is usefulas a trim control for combustion completion, by providing a certainamount of makeup air for primary combustion air, as well as a trimmethod to control the deflagrating flame front vertical location and awider lift area for unburned particles.

FIG. 19A diagrammatically shows dual stacked burner cans 190, 191 with amulti-holed air ring 192 supplying active secondary air through aone-inch gap. This burner also has four 1-inch diameter passivesecondary air holes 193 on the bottom can 191. After combustion is fullydeveloped in the first stage and the air-fuel premix is at or aboveignition temperature, we safely introduce high speed secondary air intothe oxygen-depleted environment to complete the combustion in theburner's second stage.

The effects and advantages of secondary air combustion are easilyobservable, when a single air jet with air traveling about 10 times theflame speed, V_(F), penetrates the air-starved combustion zone of anuncontained deflagrating flame completing outside of a burner enclosure.The flow pattern of the high speed secondary air is seen, as a light orclear velocity profile, really excess oxygen, dissipates and combinesinside the yellow-orange flame. We describe it as an “inside-outblowtorch flame” or an “inverted torch” with no flame in the middle, andit is an example of “inside-out completion”.

To demonstrate this two-stage combustion, a generalized andeasily-fabricated design was developed for testing. A high temperature1.5-inch ID hose 192 was assembled in a closed loop with a common feed.As visible in FIG. 19B, a series of 1/16-inch diameter holes 194 weredrilled on half inch centers around the inside of the ring at an upwardangle to provide a complete air curtain of active secondary air. The airfor this test was provided by a centrifugal blower with simple valveflow control.

The results were impressive. Providing a near-complete 360-degreeuniform, angled flow of active secondary air significantly increasedcombustion completion in the upper section, the second stage of theburner. Flame temperature increased, and flame height at the combustionchamber exit decreased. The angle, flow velocities, volume, andinteraction with the combusting particle suspension are all important asdiscussed later in this disclosure. Information from these early testsformed the basis for use of a range of methods in a variety ofembodiments discussed throughout this disclosure.

There are a variety of methods useful for active secondary air deliveryand distribution based on but not limited by the specific airdistribution system used in this embodiment. An air manifold with holesor slots may be fabricated from materials capable of withstandingtemperatures of 1500° F. or more. A single circle of machined holes maybe augmented by an array of holes above and below the plane of theactive secondary air module. The same holds true for slots or othergeometric mini-nozzle features, all of which are to be covered by ourdisclosure (see especially FIG. 26B, FIG. 35, and FIG. 42). Passivesecondary air, preferably entering the enclosure sides near the base,may also enter through the base if internal directing structures such asangled pipes or tubes control the direction of airflow to emulate thepaths taken from side entry.

For example, we have observed the benefits of providing a low speed,almost gentle type of swirl in the second combustion zone through theuse of angled active secondary air. To achieve this type of support,containment and mixing for the combusting dispersion via directioncontrolled active secondary air, a circular manifold may be utilized,comprised of an air channel/reservoir supplying machined holes, allangled identically, both rotationally (coaxially) to produce a lightswirl, and at some upward vertical angle across the flow.

An alternative, geometric passive method to the production of low speedswirl works by attaching a few stationary predominantly vertical (axial)and slightly angled “vanes” to the sides of the burner wall in protectedareas not subject to collisions of high speed particles, for example inthe recirculation zone or coaxial air paths (see FIG. 35 and FIG. 42).

A second alternative, geometric passive method to produce a low speedswirl may be accomplished by directing the flow of incoming passive oractive secondary air at the burner base from each entry orifice at thesame angle around the vertical flow axis of the burner. While these airvelocities are less than those found in the upper combustion zone withactive secondary air, beginning a low speed and gentle swirl early inthe rising combustion process does provide containment and stabilityadvantages (see FIG. 42A).

These methods extend the use of a single zone of secondary air intomultiple zones along the vertical axis of the burner, depending on thedegree of combustion control desired as in our two-stage burners. Airflow can be provided and controlled by a wide range of common industrialprocess means.

Some burner designs use a “swirler” to support and mix gases andparticles in the combustion zone, including many years in burning coalpowder. We achieve similar and perhaps more robust benefits by providingprimary combustion air through the inlet nozzle as a component of themixture stream, and supplementing with controlled secondary air.

We gain a significantly wider dynamic range of mass flow operation byour approach, as the “turn-down” ratios of swirler-based burners arelimited, primarily on the lower flow end, by an eventual loss due to thecollapse of a stable turbulent mixing by swirling support in therecirculation zone and often a minimum mass flow to maintain ignitiontemperatures in large radiant heat transfer-type furnaces and combustiondevices.

Another significant difference with swirler-supported combustion is howwe provide the total primary air for combustion. In embodiments of ourinvention, the total primary air flows through the same inlet as thefuel. Most mechanical swirler burners feed a very un-stoichiometric1-2:1 premix ratio through a centrally located nozzle and then introduceadditional primary air circumferentially around the main inlet to spinthe swirler. The “support and mixing” functions of a mechanical, airflow driven swirler are much more non-linear than the elegantly simpleapproaches found in our disclosure, and result in low flow collapse ofthat function and the subsequent low turn-down ratio performance.

The burner of FIGS. 20A and 20B utilizes similar hardware as the burnerof FIGS. 19A and 19B, but the active secondary air is fed into the lower202 of the dual stacked cans 200, 202 of this embodiment by an air ring204 through the four passive air inlet holes 206 on the sidewalls nearthe base and a 1-inch gap 208 between them for passive secondary air forthe second stage. The use of active, pressurized air flow controlthrough the lower portion of the burner may be advantageous forrecirculation, mixing, and improved early combustion completion. Activesecondary air benefits operation at lower flow rates and velocities sothat the main and recirculating air-fuel dispersion and the return ofoversized particles is not inordinately disturbed. Caution must beexercised since flows too high can cause flame out, overwhelm particlerecirculation and handling functions, and interfere with the stabilityof the stationary deflagrating flame wave front.

Secondary air, such as this, under mechanical control is referred togenerically as forced draft. Complete burner systems may be calledmechanical-draft burners when the oxidizing gas is supplied underpressure by a blower or air pressure supply device. The supply of activesecondary air for this normally passive secondary air situation in oursolid fuel burner disclosures constitutes a separate embodiment, andwould be delivered by alternate and improved means than that utilizedfor this simple demonstration.

FIG. 21A shows the dual stacked cans 210, 211, separated by a 1-inch gap212 with the addition of three active secondary air nozzles 213. Holesin the lower can provide passive secondary air. We use the term “activesecondary air” to describe a method whereby we can control the volume,pressure, and flow from an air source using various control means to aidwith accomplishment of the function of combustion completion. The airsource may be a high pressure source of compressed air or from any of avariety of blowers or other air moving means. Flows from “passivesecondary air” are set by the geometry of the air inlets, internalstructures and overall combustion chamber size parameters, and vary withthe rate of expansion of gas from combustion of the air-fuel dispersionand its mass flow.

In one embodiment, three half inch copper lines 215 feed nozzles 213comprised of an end cap with a 1/16th inch hole. As is shown in FIG.21B, an internal sectional view of the combustion enclosure, each nozzleis aimed toward the center axis of the combustion enclosure upward at a45-degree angle. The flow is high speed to accomplish second stagecompletion. Caps of various nozzle hole sizes and dispersion angles wereutilized in tests at numerous flow conditions for primary air, solidfuel mass flow rate and active secondary air flows. For a given air-fueldispersion mass flow setting, increasing active secondary air flowlowers the height of the flame 216 protruding from the burner exit bycompleting more combustion within the second stage of the burnerenclosure.

The vertical position of the deflagration flame wave front shown in FIG.21B is controlled based primarily on fluid mechanics and combustionkinetics process parameters and their mathematical response surfaces. Inthis drawing, the position of the flame front in the first stage israther high, indicating a high primary air flow velocity component ofthe air-fuel explosible dispersion mix. At lower primary air-fuel massflow settings, the height of the stationary deflagrating combustion waveis considerably lower.

FIG. 22A shows a ruggedized and taller version 220 of the originalburner design, fabricated using heavy gage steel nominal 6-inch diameterstove pipe components and cast iron pipe fittings connected to thecombustion enclosure. FIG. 22B graphically illustrates the simplifiedburner combustion system basic processes utilized for the stove pipeburner of FIG. 22A. It is the most general illustration of transitionfrom the flowing fuel dispersion to the deflagrating combustion process.

Powdered solid fuel is delivered by the auger 221 at a controlled rateinto the turbulent mixing zone 222, in this case a large “T” fitting.High velocity air flows vertically from the eXair amplifier 223, ideallybreaking up any remaining agglomerate, and entraining the fineexplosible powder particles into a dispersion for delivery verticallyinto the base of the combustion chamber generically referred to as theburner.

The air-fuel dispersion emerges from the nozzle 224, in this case ¾-inchID, in stream flow at a velocity about double the flame speed and aconcentration approximately four times stoichiometric. As this primaryair-fuel dispersion enters the combustion chamber, the BernoulliPrinciple affects the dispersion, slowing the speed and widening thefluid stream 225, decreasing air-fuel equivalence ratio.

The recirculation and turbulent mixing, shown by the two arrows 226 nearthe base, are critical to establishing and maintaining support forcombustion. Ignition establishes a stationary deflagrating flame wavefront at some vertical position above the nozzle, based on the actualprocess flow settings and fluid mechanic responses.

Vigorous combustion in the first stage reaction zone at the flame wavefront nearly instantaneously consumes the particles, with the heat ofreaction transferred from the gas to the particles by conduction in thisnarrow zone. Combustion continues until all of the available oxygenarriving in the primary air-fuel stream is consumed. Gas expansioncontinues, further adding to the turbulence and recirculation, and theunburned dispersion travels upward at combustion temperature, untilfinally encountering a fresh supply of oxidizer at the burner top,whereby the remaining fuel burns 227 to completion well above the stovepipe burner exit, beyond where we want the second stage to be.

The embodiment of FIG. 23A adds four one-inch passive secondary airholes 232 to the design of FIG. 22A. FIG. 23B graphically illustratesthe simplified burner combustion system basic processes with the addedbenefits of passive secondary air 234 at the burner base. The inducedflow of air increases combustion within the enclosures first stage area.The oxidizer supply runs out in all air-fuel mass flow except the lowestflow conditions.

The flame 236 height in FIG. 23B is shown lower than that in FIG. 22B,indicating increased combustion completion occurred within the burnerenclosure itself. An added benefit of this use of passive secondary airis increased flame front stability with improved mass-flow dynamic rangeof operation, specifically a larger turn-down ratio.

FIGS. 24A, 24B, and 24C demonstrate the relationship between the carriergas primary air flow rates at a constant high powder mass flow and theresulting flame height 242, 244, 246 emerging from the burner 240 topfor a basic six inch stove pipe burner with no secondary air flow andthree settings of low (FIG. 24A), medium (FIG. 24B), and high (FIG. 24C)primary air flow. The low primary air flow shown on the left has thetallest flame height 242. At first this seems counterintuitive. At sucha low flow, a high equivalence ratio, very little of the bulk fuelloading is completed inside the burner in the first stage. Therefore,this rather unstable flame completes upon exit at the top exhaust.

As the primary air flow rate is increased to a medium level shown inFIG. 24B, the oxidizing effect of increased carrier gas enables furthercompletion inside the burner first stage, so the exit flame height 244decreases proportionately, even though there has been no change in fuelmass flow. For FIG. 24C high primary air flow exit flame height 246above the burner top is the lowest of the three flows. This settingrequires the least vertical space to complete combustion with thehighest stability.

FIGS. 25A, 25B, and 25C illustrate the relationship of the flame height251, 252, 253 to three separate active secondary air flow rates whilethe primary mixture air flow and powdered fuel mass flow rates are heldconstant at a medium value. Passive secondary air enters through fourholes 254 of 1-inch diameter near the burner 250 base, while the activesecondary air enters through two vertical tubes 255 with a series ofspray holes in the upper part of the burner. See FIGS. 26A and 26B fortwo vertical internal tube details.

FIG. 25A shows the tallest flame height 251, as a low amount of activesecondary air is supplied, thereby requiring a substantial portion ofthe combustion to complete by virtue of room air available to the hotoxygen starved mixture at the top of the burner exit.

As active secondary air flow rates are increased in FIG. 25B and FIG.25C, the combustion requires less vertical space to complete and theexposed flame height 252 decreases accordingly. The highest secondaryair flow rate (FIG. 25C) completes all of the combustion within theburner enclosure, and is often the most desirable condition.

FIG. 26A shows the configuration and internal combustion structure for a24-inch tall by 8-inch diameter stove pipe burner 260, with 4 passivesecondary air holes 262 and two multi-holed, internal vertical air tubes264 that provide active secondary air to insure combustion completion inthe second stage inside the burner.

The performance of this embodiment is excellent, allowing for thedelivery of high BTU per hour rates while substantially completingcombustion in the burner enclosure. FIG. 26B graphically illustrates thecomplete operation of this vertical explosible powdered fuel burnersystem fed from the positive displacement powder dispersion system'smixing system through to the exhaust outlet of this 8-inch diameterburner, with a focus on the internal combustion processes fluid mechanicand kinetic processes.

Two stationary combustion fronts with different flame speeds areobserved in two distinctly separate combustion zones in our burner: thelower first zone 266 called the first stage, is a low speed process,with ignition by an igniter 261 and initial combustion near the base;and the second 268 called the second stage, a high speed combustioncompletion process driven by high speed active secondary air in theupper burner section.

Near the base is an annular combustion front at the bottom or initialsurface of the flame front. This is a fuel rich zone or volume with theinitial fuel-air (fuel-gas) dispersion arriving in the base of theburner at a velocity near double the flame speed.

There ignition is begun and sustained with continuous heating andconduction transfer from gas to particles in the reaction zone, whichquickly raises the reactant mixture above ignition temperature to asustained combustion temperature. The initial portion of the fuel airmix exiting the nozzle is not ignited until the velocity reduces andparticle dispersion diverges with the mixture at an explosibleequivalence ratio. Once heated and ignited by the flame front, the fuelin turbulent suspension at the flame front goes to completion, consumingavailable oxidizer in the region and heating neighbors.

This combustion front, following fluid mechanics principles discussedearlier in this disclosure, does not initially exhibit cross-sectionaluniformity. It is predicted and can be observed that most of the initialburning takes place in a hollow, diverging volume characterized by highspeed, rich and unignited flow in the center, with lower speed turbulentcombustion wrapped around and forming the outside of the burning volume.

In this initial fuel rich zone, the powder-air stream is partiallyburned crossing the ignition wave front of the flame. This burningcontinues to consume and complete particle combustion until the locallyavailable oxygen is spent. The remainder of the unburned or partiallyburned fuel suspension, raised above ignition temperature, moves awayfrom the first stage zone combined with nitrogen gas, CO₂, and zero freeoxygen.

Ideally, this superheated fuel rich, oxygen depleted annular powdercloud proceeds into the second stage combustion zone where it encountersa high speed uniform and concentric oxygen rich flow of active secondaryair, rapidly burning completely.

Room temperature secondary air at reasonably high velocity and/or flowrates can enter this more stable second stage combustion zone,superheated greater than the combustion temperature, with an intense airstream at much higher velocity without flame blow out than is possiblein the first stage initial combustion zone.

However, it is important that the active secondary air forces, whichimpact the hot oxygen starved dispersion, are not so energetic or widelyoff axis that they overwhelm the natural turbulent flow in this secondstage combustion zone, knocking these hot particles out of suspensionand thereby causing agglomeration or impact with the combustionenclosure walls. Proper choice of flows, velocities, and distributiontechnique and direction angles is important, as anyone skilled in theart can appreciate. These principles are extensible to a wide variety ofsecondary air delivery configurations used for two-stage combustion, andare not intended to be limiting.

It should be understood that the size of the burner combustion enclosurevolume is primarily dictated by the BTU/hour rate planned for normaloperation, even though we have a remarkable and uniquely high dynamicrange of operation for a solid fuel combustion device, typically a 10:1turndown ratio. For a burner this large, it makes sense that at low flowrates the width of the 8-inch burner is close enough to “free space”that its performance declines. This 200,000 BTU/hour embodiment istherefore not normally intended for continuous operation in the10-20,000 BTU/hour range.

For flow rates sufficiently low, the vertical walls of a burner have toolittle effect on the flow patterns, since they are effectively“infinitely” far away. Specifically for low air-fuel explosible mixtureflow rates, this wide burner provides less flow resistance to thedispersion stream, hence will generate less turbulence by shear,resulting in a decrease of mixing, recirculation and flame re-ignition,thereby bringing a decrease in temperature to the lower part of theenclosure.

At low flow conditions, the Bernoulli effect continues to operate, yetis substantially reduced in effectiveness. With this wider outletopening for a given height burner, we lose the various benefits of fluegas and flame recirculation as mentioned above. Simply put, at low flowrates the 8-inch burner is close enough to “free space” that it stopsworking well compared to more appropriate mechanical designs.

FIG. 27 shows a very robust system of supplying active secondary air tothe second stage upper combustion zone of a 6-inch steel stove pipeburner 270. In one embodiment (not shown), three half inch copper tubesare run on the outside of the burner steel enclosure at a separation of120 degrees around the vertical axis. Each tube is closed off with anend cap on a 45-degree street elbow, having a 1/16th inch hole for thenozzle orifice. The three nozzle end caps penetrate the side of thecombustion chamber sidewall in a similar manner to the followingembodiment.

The embodiment shown in FIG. 27 utilizes four copper active secondaryair tubes 272, each ⅜-inch in diameter, running up the outside of thestove pipe combustion enclosure. These tubes are bent at about 75degrees to the vertical, with nozzle end caps penetrating the enclosure.Both include passive secondary air holes 274 near the base. We found airjets located at or near the circumference of the burner aimed inward atabout 15 degrees off the burner flow axis performed well.

For both configurations, a wide range of control was available to adjusta tenfold range of BTU/hour energy conversion rates (20,000 to 200,000BTU per hour) with stable deflagrating flame wave front combustion andprotruding flame heights from near zero to two feet above exit possible.This burner system is a highly efficient and controllable combustionsystem for our powdered fuel. It is easy, from a cold start, to raisethe internal operating temperatures in the second combustion zone above1900° F. in under a minute.

There is obviously a complete response surface that describes thiscombustion process and secondary air introduction based on fluidmechanics and combustion kinetics. Therefore, our disclosure is notlimited to these specific angles, secondary air flow rates, pressures,number of jets, vertical location, and jet nozzle hole diameterscombined with burner diameter and air-fuel flow rates.

Maintaining Explosible Conditions and Avoiding Agglomeration

Various means and methods may be used to maintain explosible conditionsand avoid issues of agglomeration for the explosible powder in a burnerof the present invention. An explosible dispersion is developed and keptstable by a number of techniques with special emphasis on three:maintaining the turbulent energy level inside burner; feeding thedispersion of individual explosible particles into the combustionchamber at a sufficient velocity with adequate primary and secondary airto insure a non-lazy suspension of all fuel particles for combustion;and avoidance of collision with structures or other particles whichencourage agglomeration formation and growth.

The explosible dispersion begins at the exit of the auger within thepositive displacement powder dispersion delivery means (PDPD), althoughproblems with particle size maintenance issues may begin far upstream ofthis location and can possibly continue downstream as well.

After manufacturing, the powdered fuel is subject to the forces ofhandling, storage and metering prior to final air-fuel mixing, ignitionand combustion. These processes introduce mechanical shear, whichcombines with humidity and the fibrous nature of biomass, to produceagglomerate, larger particles comprised of many small ones.

As a result of the comparatively “large size” of agglomerates, theypresent problems as they do not conform to the requirements of singlephase combustion in that they do not flash burn instantly, they are slowto heat, do not burn completely, but may burn partially and form a charcoating, plus may fall out of suspension, due to their mass andaerodynamic properties. If recycled soon and gracefully inside a burner,they may be entrained and burned. If recycled improperly as large, lowvelocity particles into the high velocity powder dispersion stream, theymay incur multiple collisions, often building in size like a rollingsnowball.

In the series (FIG. 28 through FIG. 39) that follows, we discloseembodiments of a recycle consuming burner shown especially in FIG. 37.Various means for collection of unburnable particles and ash are alsocovered. It is anticipated that this integrated approach to managingrecycle, agglomeration issues, some particle size reduction andcombustion residue with powdered solid fuels is unique for powder fuelcombustion.

FIG. 28 shows internal flow and recirculation patterns in a 6-inch steelstove pipe burner 280 fitted with a 30-degree sloped cone insert 281 atthe base to perform unburned particle and agglomerate recirculation andmixing. The internal flow and turbulent recirculation patterns are shownby the arrows in this totally enclosed burner without the benefits ofpassive secondary air. The hole in the bottom of the cone is wide enoughto allow the vertical primary air-fuel dispersion to flow upward,unimpeded.

Any type of particles, from powder to oversized powder particles toagglomerate 282 that is caught up in the recirculation turbulence andfalls out of suspension is gently directed back toward the vertical flowstream 283 for re-entrainment.

FIG. 29A shows an alternate agglomerate recirculation and mixingembodiment strategy employing four passive secondary air inlet holes 291combined with vibration 292 in a similar 6-inch stove pipe burner 290.The goal is to compare the effectiveness with the 30-degree coneapproach used in FIG. 28. The internal flow and turbulent recirculationpatterns are shown by the arrows in this burner with an improvement invertical flow and stronger recirculation turbulent mixing.

A vibration apparatus 292 is attached to the bottom of the burner,causing any particles or agglomerate fallout to dance on the bottom ofthe burner. This vertical vibration energy, when combined with the crossburner “sweeping” function performed by passive secondary air,encourages particle and agglomerate fallout to move toward and beentrained by the high velocity vertical premix dispersion flow.

FIG. 29B is a magnified view of the lower right section of the burner ofFIG. 29A, showing how the “sweeper” air recirculates unburned “dancingparticles” comprised of agglomerate, individual or oversized particles293. The use of a vibration apparatus 292 encourages this verticaldancing or bouncing, so the wide range of flow from the passivesecondary air holes can move any fallout back toward the high premixflow for return to the stationary deflagrating wave for combustion.

The embodiment of FIG. 30A combines the strengths of methods detailed inFIG. 28 and FIG. 29B and improves on them by raising a wide hole versionof the 30-degree cone design upwards enough to allow passive secondaryair 301 entering from below through four 1-inch holes 302 to assist withlifting and entraining fallout particles 303 gathered by the cone backtoward the stationary deflagrating flame wave front.

Burner design 300 combines both recirculation and passive secondary airtogether to further improve functionality. With more oxygen available inthe first stage combustion zone, the resulting flame height 306 at theburner top exit is therefore decreased.

After exiting a metering device, an embodiment of our system designdrops the fuel including some aforementioned agglomerate into asufficiently turbulent air stream. This turbulence de-agglomerates thesubstantial majority, dispersing it in a manner sufficient to result innear complete combustion (99.8%+) in a fraction of a second.

Sufficient turbulence to de-agglomerate most powders withinspecification (i.e. not overly moist) is produced in the mixing “T”fitting 305 called the mixing zone, located between the auger output 306and air amplifying eXair eductor combination 307 used with ourhorizontal auger PDPD feed system for both horizontal and verticalburners. With horizontal auger PDPD feeds into near-horizontal burnerconfigurations, we utilize an additional air jet, produced by sendingpressurized air through a 1/16-inch hole into the falling auger outputstream (see FIG. 40A).

FIG. 30B is a magnified view of the bottom section of the burner of FIG.30A, with arrows showing how the recirculation flow patterns above theelevated 30-degree wide mouth cone 304 encourage particles 303 backtowards the vertical and rising premix dispersion. These functionscombine with flow from the passive secondary air holes 302 which risescoaxially around the dispersion, concentrated by the wide hole in thecone. By focusing and restricting the area for this secondary flow, itsability to lift particles and small agglomerate for recirculation issignificantly improved.

It should be noted that the use of four 1-inch holes as an inlet supplyfor passive secondary air is only one of numerous methods to accomplishthe same function. The same applies to air flow supply methods and meansemployed if conversion from passive to low flow active secondary air istechnically warranted.

FIG. 31A shows a burner of the present invention with an ultrasonicagglomerate lump dispersing screen system 311 in the lower section ofthe burner enclosure. The vertical flowing air-fuel dispersion stream312 flows through the center of the screen 311 towards the stationaryflame wave front 313.

As background, depending on the combined characteristics of the fuel,forces of mechanical shear encountered in handling, humidity plusposition of the clumps dropping into the turbulent mixing zone, someagglomerate may resist complete destruction by the previously disclosedintensely turbulent air stream and configuration of the mixing zonealone. Rather they can become smaller, smoother and more roundedagglomerate.

The vast majority of the agglomerate problem may be solved by judicioususe of an agglomerate destructor screen, shelf or table-like structure,powered by an ultrasonic transducer driven by advanced signal processingtechniques, including a multi-frequency spread spectrum type of signal.The screen does not necessarily have to be quite as fine as the finalmesh size of the original individual particles, just with small enoughopenings to impart adequate energy to the agglomerates to disintegratethem into their original individual particle constituents, whichsubsequently flow through the screen unimpeded.

If any agglomerates are formed downstream of this deagglomeration stagethrough collisions, such as particle-to-particle or with flow conduitwalls and fittings, they must be dealt with separately in the burnerthrough various methods described in our unique and inventive methodsfor agglomerate destruction, collection and recirculation found in thisdisclosure. These techniques are outlined in descriptions of severalburner design configurations which follow, and in combination,constitute further inventive disclosure.

A top view example of a general design for an ultrasonic agglomeratelump destructing screen appears like the head of a tennis racket. Thehole 314 in the middle is of adequate diameter for the still essentiallylaminar vertical high speed primary air-fuel dispersion to flow throughwithout particle collisions. The screen is shown mounted by fourvibration isolation points 315 inside the 6-inch diameter burnerenclosure 310, and connected to an ultrasonic transducer 316.

FIG. 32 shows the combination of an elevated 30-degree wide holed cone322 with an ultrasonic lump destruction screen 324 driven by atransducer 328 to comprise an agglomerate recirculation and dispersionsystem, augmented by baffled passive secondary air in a 6-inch verticalburner 320. Particles 321 from agglomerate 326 fall through theultrasonic destructor screen, and through the mechanisms of burnerenclosure low frequency vibration and internal air recirculation, findtheir way to the edge of the cone's central hole 323 where they areswept up and transported as fresh reactant back towards the reactionzone 327 by a combination of eduction from the high speed air-fuelstream and the up-rushing lift from secondary air flow 325 around theflow stream 329.

FIG. 33 shows one type of “top hat” flow restricting reducer 332 mountedatop the 6-inch burner of FIG. 27. This 8-inch to 6-inch reducer setatop a 6-inch burner 330 allows a passive or active tertiary air flow336 to aid with final combustion completion, while still offering someflow restriction on the burner exit throat 334.

It was observed that the presence of this reducer, also called a“densifire” and “intensifire”, reduced the exit 334 flame height,affected the flame color, increased the temperature and internalcirculation, and increased the sound intensity. This burner was roaringwhite hot flame at 334, especially with the aid of active secondary air338 and passive secondary air through holes at the base. A reducedoutlet area likely improves the turbulent recirculation functions, as itproduces a higher blockage ratio, a non-dimensional ratio of burnercross sectional area to discharge throat area. Reducer 332 outletrestriction may be particularly useful to maintain stability, bypreventing collapse of recirculation during operation with low BTU/hourprimary air-fuel flows in larger diameter burners.

An alternative embodiment to further restrict outlet flow uses a tightfitting 6-inch to 4-inch reducer 334, thereby eliminating the simple oneinch circular tertiary air inlet 336, which can be accomplished bydifferent means if desired.

FIG. 34A depicts a early horizontal combustion and powder dispersionfeed system using a four inch can burner 340 with passive secondary air.The powder is fed by an auger 344 into a tee fitting where it drops downthrough conduit 346 and into an inverted tee fitting. There itencounters high pressure air from inlet 347 which breaks up much of theagglomerate from screw feeding, and is turbulently mixed with sufficientprimary air to supply the eXair ½-inch air amplifier 348. The dispersionis further improved by this operation, with more agglomerate breakup,then sent in a fuel rich flow dispersion toward the nozzle 343 where itenters the simple combustion enclosure and is ignited. FIG. 34B is asectional view of the four-inch burner can 340 showing one of fourpassive secondary air holes 342 and primary air-fuel connection 343 fromthe powder dispersion feed system.

FIG. 35 diagrams the internal geometric and combustion structure of a4-inch horizontal burner assembly 350 with novel passive secondary airmanagement accomplished by dual coaxial enclosures 350, 352. Many ofthese concepts are present in today's high capacity 1 MM BTU/hrhorizontal burner. The primary air-fuel mixture is fed into the nozzlelocated in the 3.5-inch diameter inner enclosure 352 as detailed inFIGS. 34A and 34B. Ignition occurs a few inches downstream from thenozzle and combustion proceeds into the outer 18-inch long six inchdiameter steel tube 350.

Passive secondary air 359 finds its source in large air inlets 356 and358, and is guided around the first stage reaction zone protected byenclosure 352 through a circular channel 354 between the inner and outerenclosure. Along that path are several air directing structures. 355 isan angled cowling to direct the secondary air toward the second stagecombustion zone. 355 a is similar to 355 and has the same function, buthas a less abrupt angle. There are optionally several angled vanestructures 355 b, located around the inside of enclosure 350 used toproduce “swirl” in the second stage secondary air. They are angled inthe circular channel like stationary fan blades, imparting rotation tothe air flow. Airflow 357 is our classic passive secondary air feedingthe first stage. The remaining combustion takes place at the exit of thelong tube horizontal burner, sourced by outside air, and the flame 351curls upward as final gas expansion takes place.

This embodiment is easily extensible to the use of active secondary airthrough inlets 356 and 358, and controlled by choice of the gap 354between the two enclosures.

FIG. 36 shows a 4-inch diameter horizontal burner 360, fabricated insteel with four active secondary air tubes 362 intended for prototypetesting as a direct retrofit into an oil-fired furnace. It utilizespassive secondary air through four holes 363 to achieve a tall,horizontal version of the vertical burner in FIG. 27. This burner hasbeen highly tested and is ready for prototype trials in a formerlyoil-fired furnace, delivering 200,000 BTU/hour.

FIG. 37A graphically depicts a basic recycle collecting horizontalburner 370, tilted up 372 preferably by about 3 to 5 degrees abovehorizontal for agglomerate and oversized particle collection. A vibratoror ultrasonic transducer 374 causes high-frequency vibration of theburner, which allow agglomerates and oversized particles 376 to moveback in the burner toward an opening and into a collection vessel 378.The unburned fuel collected in the collection vessel 378 may bereprocessed to an explosible form or discarded.

FIG. 37B graphically depicts a basic recycle collecting horizontalburner 370, tilted downward 373 preferably by about 3 to 5 degrees belowhorizontal for front end agglomerate and oversized particle collection.A vibrator or ultrasonic transducer 374 causes high-frequency vibrationof the burner, which allow agglomerates and oversized particles to moveforward in the burner toward an opening 377 and into a collection vessel378. The opening 377 is preferably at the end of the burner but may belocated back from the end slightly. The opening 377 may also be formedas a spout to better direct the agglomerates and oversized particle tothe collection vessel 378. The collection vessel 378 is preferably largeenough to require only occasional emptying. Unburned fuel collected inthe collection vessel 378 may be reprocessed to an explosible form ordiscarded.

FIG. 38 shows a more automated recycle consuming gravity collectingclosed loop solid fuel horizontal burner system with agglomerate andoversized particle reprocessing and reintroduction into the air-fueldispersion. In this embodiment, the burner 380 is tilted upward 382,preferably by about 3 to 5 degrees. A vibrating or ultrasonic transducer384 causes high-frequency vibration of the burner, which nudgeagglomerates and oversized particles to move back in the burner toward arecycle system. The particles fall toward a recycling device 386, whichsends the particles along a recycle path 388 feeding into the mainpowdered fuel dispersion stream, or into the mixing system. The recyclesystem preferably provides agitation or grinding to break theagglomerates into small particles.

FIG. 39A shows a recycle agglomerate destructing horizontal burner 390using an ultrasonic driven screen 394 for deagglomeration. The burner ispreferably tilted up 392 by about 3 to 5 degrees. An ultrasonictransducer 396 causes high-frequency vibration of the screen 394 tobreak up agglomerates falling onto it and vibration of the burner 390,which allows de-agglomerated particles to move back in the burner towardan active secondary air flow source 398, which recycles the particlesback toward the main powdered fuel dispersion stream.

FIG. 39B shows an end view of the recycle agglomerate destructinghorizontal burner 390 showing the presence of the ultrasonic screen 394across the lower portion of the burner and a trough for particlecollection and movement.

FIG. 40A shows a piping drawing detail of a preferred mixing zone andinfeed for horizontal burners. A horizontal auger 403 feeds powderedfuel which mixes with oxidizing gas entering at hole 401. The dispersionthen travels down a vertical conduit 405 to a horizontal conduit 404,where additional oxidizing gas is added at point 402 before thedispersion travels through an eXair amplifier 400. The burner of FIG.40B adds an ultrasonic deagglomeration screen with an ultrasonictransducer 406 in the mixing zone 405 to the piping interconnectiondrawing of FIG. 40A for horizontal burners.

FIG. 41 graphically shows a wide range, upward vertical to horizontal todownward vertical orientations possible for the disclosed solid fuelexplosible powder burners. About a 0-degree orientation burner 410,about an 45-degree orientation burner 412, about a 90-degree orientationburner 414, about a minus 90-degree orientation burner 416, and about aminus 45-degree orientation burner 418 of the present invention areshown in FIG. 41.

Propane burners work at all angles. Ours will too although eachorientation shown brings special requirements. It is important tounderstand the performance differences and advantages of eachorientation, plus the challenges and opportunities.

Use of a vertical or near vertical burner provides the opportunity for agiven burner design to 1) run at lower flow rates and 2) perform with alarger controllable dynamic range (turndown ratio) than may beaccomplished with a near horizontal or downward burner. The majorperformance difference is due primarily to the challenge of maintainingsuspension of the fuel-air mix with very significant particle and gasdensity differences encountered (perhaps >500:1) when feeding andsuspending powdered fuels.

Use of a vertical configuration with various types of vibration alsoallows for operation with larger particles that typically fall out ofsuspension more rapidly in horizontal type burners, which do notencounter the same benefits of symmetrical vibration and recirculationpatterns of non-explosible particles afforded by the vertical designs asdisclosed elsewhere. Near vertical burners can work with a largerfraction of non-explosible particles than horizontal ones, therebyaffording the opportunity to utilize lower cost powdered fuels. Aspecification for what we call “sloppy fuel” may include a manageablysmall portion of marginally explosible, slightly oversize or oblongparticles by choice. Certain applications can easily accommodate thecollection and housekeeping issues, such as farms and some industrialuses where the lower cost of fuel pays out.

Downward burners have to fight gravity to keep the combustion reactantssuspended and under control. Explosible mixture feed velocity is loweredto maintain the stationary deflagration wave at about the same location.More work is performed by combustion gas expansion and its tendency torise. Active secondary air near the burner base becomes important forfirst stage turbulent mixing at lower dispersion stream infeedvelocities. Downward burners require a finer with the least oversizefuel particles, unless used in a “sloppy fuel” application.

Opportunities exist for near horizontal burners too, provided that anyballistic issues for agglomerate, over sized or high aspect ratio properdiameter particles can be managed using the techniques disclosed herein.Combined with good fuel management and choices, near horizontal burnersstill offer a large operating window and options to minimize some of theproblems encountered with imperfect fuels.

FIG. 42A depicts our current 1 MM BTU/hr burner slated for test on agrain drier. Many of current design concepts evolved from FIG. 35 burnerdesign principles. This coaxial unit uses an inner first stagecombustion enclosure 420 with passive secondary air holes 421. Activesecondary air 422 is fed through one or more supply hoses 422 a to acircular plenum 423 formed by larger outer enclosure 424 and an outercollar 423. Heading for the second stage combustion completion zone, theactive secondary air is fed through an angled directing slot in circularinsert 425 a and emerges at speeds up to 10× times the powder flamespeed to mix the hot particles and supply necessary oxidizer forcombustion completion. Optional swirl vanes circumferentially mounted inthe coaxial air channel as in FIG. 35 may be used for additional secondstage combustion zone support.

FIG. 42B is uses the FIG. 42A design and simply adds active secondaryair to the first stage burner enclosure sides near the nozzle end topromote better control of the recirculation and flame support functions.One or more supply hoses 428 feed the oxidizing and support air 421 intoa collar or plenum 428 a for distribution.

FIG. 43 is a system level block diagram of a furnace for heating withexplosible powder fuel. A PDPD system, comprised of an oxidizing gasfeed 4310, a powder fuel feed 4312 with hopper 4314 and a mix zone 4316,feeds the dispersion to the burner 4318, which exhausts into the furnaceheat exchanger 4320. Flue gas 4321 continues through a heat recoveryexchanger 4322, then a particle filtration system 4324 where airentrained ash is removed and stored 4326 for disposal. The flue gasexhausts to atmosphere 4335. A heating fluid 4328 circulates through thefurnace heat exchanger 4230, then to the main heat load 4330, returninglow temperature fluid back through a heat recovery exchanger 4322. Theburner system, external powder storage 4332 and flue gas particlefiltration systems 4324 comprise a unique to adaptation to a furnace forexplosible powder fuel direct energy conversion to produce heat.

The most important point to understand about the prior art, whendifferentiating a burner design and combustion regimen of the presentinvention from others appearing similar at first glance, is exactly howprior combustion system process operations and burner design conceptsconclusively demonstrate again and again the fact that each of thoseprior art approaches relies upon the art of two-phase combustion regimesregardless of the solid fuel types and particle size distributions.

Fuel parameters must be mentioned since they impact on techniques tomaintain a completely explosible combustion regime and ties to burnerdesign choices and operation.

Particle size is important, and the “quality” of that distributiondetermines the ability of the burner combustion process to run mostefficiently and cleanly in the explosible zone, our claimed region ofoperation. The primary focus is on minimizing oversize particles, oneswith a diameter/length over the explosibility threshold for thatmaterial (˜200 microns for wood), when it comes to the particle sizedistribution's upper/outer limit composition as a percentage of theentire distribution.

Both the fuel raw material type and powder production process used aremajor factors for the final manufactured powder fuel's adherence tospecification. It is desirable to have a fuel, that when combusted,produces no “sparklers” or “rocket particles”, which may result inbyproducts requiring disposal. These particles are in fact oversized andwill comprise some very small quantity of the overall substantiallyexplosible powdered fuel, in spite of best manufacturing efforts.Decisions on the final grade specification will be based on the burnersystem performance versus manufacturing system economic models.

The larger the particles, the more quickly they settle out and the moreattention dispersion support requires. The transition to viability ofsingle-phase combustion behavior occurs at the explosible boundary with200+ micron hardwood particles at STP conditions. With particles veryclose to the transition diameter, we still have to pay a lot ofattention to ensure the flow behaves properly. By comparison, if we wereutilizing 10 micron particles, maintenance of good dispersion flowconditions would be relatively easy, but the cost of particle sizereduction by grinding would be a great deal higher. 80-mesh (177 micron)pine burns better in the vertical burner than in the horizontal, sinceit contains larger particles. 200-mesh (74 micron) hardwood burns greatin both.

A mixed flow of 200 and 100 micron diameter particles will beexplosible, if the air-fuel equivalence ratio is in the correct range.With particles on the large side close to the explosible transitionline, getting a proper dispersion of air and powder is more difficult.As a rule of thumb, as particle size is slowly reduced, explosiblebehavior shows up long before it becomes easy to keep the population ofparticles in suspension.

Usually, a distribution of particles is discussed based on percentagesof its population that are above or below some threshold value, ordistribution mean, median or mode. However, when it comes toestablishing and maintaining an explosible dispersion, it's often moreuseful to discuss the distribution in terms of weight percentages.

We define a dimensionless number χ_(w) which equals the explosibleweight W_(e) of the fuel distribution (those particles less than the200+ microns explosible in diameter), divided by the total weight W_(t)of all particles in the population. The goal is to only work withdistributions where We approaches or equals 1. The smaller the number,the more difficult the distribution, hence dispersion, is to work with.χ_(w) =W _(e) /W _(t), where χ_(w)→1

In our burners, oversize particles literally become “ballast” and oftena nuisance when attempting to work exclusively with substantiallyexplosible mixtures. We are going out of our way to create and suspend asolid fuel air suspension or dispersion, whereby it will burn as asingle-phase mixture, then ignite it.

Burner System Embodiments of Powdered Fuel-Powered Devices

The burner systems and methods disclosed herein may be applied forheating or energy purposes to a variety of device applicationsincluding, but not limited to, furnaces, engines, boilers, grain dryers,clothes dryers, hot water heaters, combined furnace/water heaters, hotair balloons, space heaters, wood burning stoves, gas fireplaces, gasturbines and electrical generators, forced hot air heating systems,forced hot water heating systems, forced steam heating systems, andradiant heating systems. Additional devices include ovens, absorptionchillers, ammonia cycle refrigeration, patio heaters, heating torches,controllable fire pits, continuous water heaters, booster and inlinewater heaters, yellow flame gas log sets, inserts, freestanding stovesand built in zero clearance fireplaces, radiating stoves and furnaces,outdoor wood boilers, industrial furnaces and boilers, corn stoves,pellet stoves, coal stoves, entertainment controllable torches,pyrotechnic type displays, and steam engines by replacing orsupplementing existing combustion devices with powder burning devices.

Kit for Retrofit Application for Deflagration of Powdered Fuels

The various horizontal and near-horizontal burners described in thisdisclosure are intended to be utilized first as a retrofit for existingoil fired furnaces, where a nominal four inch receiving and mountingcollar system is common. A preferred embodiment for this application istested and running at this time and is best described by FIG. 36. Anyissues with agglomerate may be dealt with using one or more of thetechniques described in FIG. 37A through FIG. 40B for horizontalburners, as well as solutions developed with vertical burners in minddepicted in FIGS. 28 through 32.

Application of an apparatus for combustion of a fluent fuel may be madeto many self-contained devices, but are often part of, or used inconnection with, heat-consuming apparatus, such as heating furnaces andboilers. In many of these fields of fluent fuel combustion, our burnerapparatus may be considered a subsystem detail of a larger end productentity. A burner of the present invention may be adapted for use in anytypes of furnaces, boilers, and other heating systems such as but notlimited to those today typically fueled by oil, natural gas, and LP gas.A burner of the present invention may also be adapted for use in anexisting system of any of the applications mentioned above.

Our wide range of burner technology, means, methods, and apparatuses arealso intended to be adapted to existing heat exchange furnace and othersystems, or designed for installation in new heat exchange furnaces andother systems, as described above.

The contents of all references, issued patents, and published patentapplications cited throughout this application are hereby incorporatedby reference.

Accordingly, it is to be understood that the embodiments of theinvention herein described are merely illustrative of the application ofthe principles of the invention. Reference herein to details of theillustrated embodiments is not intended to limit the scope of theclaims, which themselves recite those features regarded as essential tothe invention.

What is claimed is:
 1. A method of combustion of an explosible powderedsolid fuel, the method comprising: a) mixing a flow of a carrier gas anda plurality of solid particles of the explosible powdered solid fuelhaving a size distribution substantially in an explosible size range forthe solid particles to form a moving stream of a dispersion of the solidparticles of the explosible powdered solid fuel in the carrier gas; b)directing the moving stream into a first enclosure of a burner throughan inlet nozzle to form an explosible dispersion in the first enclosuremoving at a flame speed of the explosible dispersion, the burnercomprising: i) a first enclosure sidewall having a closed inlet end andan open exhaust end opposite the closed inlet end, the first enclosuresidewall forming the first enclosure having a substantially cylindricalshape, the first enclosure including a first combustion stage; ii) theinlet nozzle coupled to the output of the mixing zone and having an exitinto the first enclosure at the closed inlet end of the first enclosure,wherein the inlet nozzle opens unobstructedly and freely into the firstenclosure to deliver the moving stream of the dispersion of the solidparticles in the carrier gas to the first enclosure; iii) an igniterlocated in the first enclosure downstream from the inlet nozzle toinitially ignite the dispersion; iv) a first flow-alteringdiscontinuity, formed by a first diameter step increase from an innerdiameter of the inlet nozzle to an inner diameter of the first enclosuresidewall, with a first discontinuity ratio of the inner diameter of thefirst enclosure sidewall to the inner diameter of the inlet nozzle in arange of 4.67 to 12; v) at least one first radially-symmetricalflame-stabilizing opening permitting flow of a first controlledsecondary gas through the first radially-symmetrical flame-stabilizingopening into the first enclosure, the first radially-symmetricalflame-stabilizing opening being located at the closed inlet end of thefirst enclosure, wherein the first radially-symmetricalflame-stabilizing opening is formed to direct the first controlledsecondary gas along and then outward from a flow direction axis of thedispersion and recirculating along the first enclosure sidewall; vi) asecond enclosure sidewall having an inlet end and an open exhaust endopposite the inlet end, the second enclosure sidewall extendingdownstream beyond the open exhaust end of the first enclosure sidewallto the open exhaust end of the second enclosure sidewall, the secondenclosure sidewall forming a second enclosure having a substantiallycylindrical shape, the second enclosure including a second combustionstage; vii) a second flow-altering discontinuity, formed by a seconddiameter step increase from the inner diameter of the first enclosuresidewall to an inner diameter of the second enclosure sidewall, with asecond discontinuity ratio of the inner diameter of the second enclosuresidewall to the inner diameter of the first enclosure sidewall being ina range of 1.14 to 2.29 at the open exhaust end of the first enclosuresidewall; and viii) at least one second radially-symmetricalflame-stabilizing opening permitting flow of a second controlledsecondary gas through the second radially-symmetrical flame-stabilizingopening, the second radially-symmetrical flame-stabilizing opening beinglocated downstream of the open exhaust end of the first enclosuresidewall, wherein the second radially-symmetrical flame-stabilizingopening is formed to initially direct a radially-symmetrical stream ofthe second controlled secondary gas inward in the second enclosure in adirection non-parallel to a central axis of the second enclosure,wherein the second controlled secondary gas is not externallypre-heated, is pressurized, and is actively provided at a velocity up to10 times the flame speed; c) igniting the explosible dispersioninstantaneously with the igniter; d) supplying the first controlledsecondary gas to the first enclosure; e) actively supplying the secondcontrolled secondary gas to the second enclosure; and f) controlling theflow of the carrier gas, flow of the first controlled secondary gas,flow of the second controlled secondary gas, and a feed rate of thesolid particles to maintain an equivalence ratio in the explosible rangedownstream of the inlet nozzle, thereby maintaining an instantaneouslyself-sustaining stable stationary flame front of combustion of theexplosible dispersion within the first enclosure.
 2. The method of claim1, wherein the first controlled secondary gas is supplied to the burnerthrough at least one secondary inlet to the first enclosure between theclosed inlet end and the open exhaust end for turbulence-inducing flowin a direction non-parallel to the first enclosure sidewall.
 3. Themethod of claim 1, further comprising monitoring the burner using atleast one sensor selected from the group consisting of an intaketemperature sensor, an exhaust temperature sensor, an intake gas sensor,exhaust gas sensor, a mass airflow sensor, an air/fuel ratio sensor, afuel flow sensor, an oxygen sensor, a carbon monoxide sensor, a powdersupply sensor, an acoustic sensor, a powder sensor, a vacuum sensor, apressure sensor, a position sensor, a powder feed speed sensor, a staticcharge sensor, a humidity sensor, a moisture sensor, a particle sizesensor, an optical sensor for sensing a presence of the flame, and anycombination of these sensors and controlling at least one parameter ofthe burner based on a reading from the sensor.
 4. The method of claim 1,further comprising regulating a heat output of the burner by controllinga rate of flow of the carrier gas, a rate of flow of the firstcontrolled secondary gas, a rate of flow of the second controlledsecondary gas, or the feed rate of the solid particles.
 5. The method ofclaim 1, wherein the at least one burner comprises a plurality ofburners and the method further comprising regulating a total heat outputby the plurality of burners by controlling the flow of the carrier gas,a rate of flow of the first controlled secondary gas, a flow of thesecond controlled secondary gas, and the feed rate of the solidparticles to at least one of the plurality of burners.
 6. The method ofclaim 1, further comprising converting energy produced by the combustionthrough the operation of a device selected from the group consisting ofovens, absorption chillers, ammonia cycle refrigeration, patio heaters,heating torches, controllable fire pits, continuous water heaters,booster and inline water heaters, yellow flame gas log sets, inserts,freestanding stoves, built in zero clearance fireplaces, radiatingstoves and furnaces, outdoor wood boilers, industrial furnaces andboilers, corn stoves, pellet stoves, coal stoves, entertainmentcontrollable torches, pyrotechnic type displays, and steam engines byreplacing or supplementing existing combustion devices with the burner.7. The method of claim 1, wherein the first controlled secondary gas ispassively provided.
 8. The method of claim 1, wherein the firstcontrolled secondary gas is actively provided.
 9. The method of claim 1,wherein: the first flow-altering discontinuity and the firstradially-symmetrical flame-stabilizing opening provide, in combination,turbulent mixing of the carrier gas, the first controlled secondary gas,and the solid particles, to promote initial particle heating, to shortena length of a pre-heat zone, to promote flow of the first controlledsecondary gas, and to provide combustion flame stabilization; the firstenclosure is designed such that when the moving stream of the solidparticles in the carrier gas enters the first enclosure from the inletnozzle, a backflow causes the moving stream to diverge, thereby formingan explosible dispersion having a symmetrical mushroom-like shape andslowing to a flame speed of the explosible dispersion by virtue of thefirst flow-altering discontinuity and the first radially-symmetricalflame-stabilizing opening, wherein through turbulent recirculation mixeswith the first controlled secondary gas to form the explosibledispersion at an equivalence ratio in an explosible range, and whereinthe ignition source initially ignites the explosible dispersion; astream flow of the moving stream is regulatable to maintain aninstantaneously stable stationary deflagrating flame wave front bycombustion of the explosible dispersion, thermally sustained withnon-radiative, gas-to-particle, conductive heat transfer, whileemulating fluid mechanics of single phase gas combustion; the secondflow-altering discontinuity and the second controlled secondary gasentering through the second radially-symmetrical flame-stabilizingopening provide, in combination, an induced backflow recirculation tofurther combust any remaining solid particles and to promote a turbulentflow inside the second combustion stage to mix an efflux from the firstenclosure with the second controlled secondary gas, to increase a rateof heating of the second controlled secondary gas by the efflux, todecrease a length of a final burnout zone, to sustain flamestabilization, and to insure combustion completion; the carrier gas, thefirst controlled secondary gas, and the second controlled secondary gascomprise, in combination, sufficient oxidizing gas to completely combustthe solid particles; and heating of the solid particles in the firststage relies on gas-to-particle conduction and heating of the secondcontrolled secondary gas in the second stage relies on particle-to-gasconduction, not on any thermal refractory structure for heat retentionor radiative heating or any external thermal energy, and flamestabilization does not rely on any active mechanical swirl inducingapparatus common to cyclonic and vortex suspension systems forincreasing path length and residence time.
 10. The method of claim 1,wherein the burner further comprises at least one passive stationaryflow-altering structure located in the first enclosure to induce swirlor turbulence to the moving stream.
 11. The method of claim 1, whereinthe first and second enclosures are vertically oriented such that theopen exhaust end of the second enclosure is at the top.
 12. The methodof claim 1, wherein the first and second enclosures are non-verticallyoriented.
 13. The method of claim 1, wherein the burner furthercomprises a conical arrangement at the closed inlet end of the firstenclosure for catching and recycling unburned solid particles into themoving stream.
 14. The method of claim 1, wherein at least 95% of thesolid particles have a diameter in the explosible range, which is lessthan about 200 microns for wood.
 15. The method of claim 1, wherein theburner further comprises a de-agglomerization system comprising a screenlocated in the first enclosure and a vibrating transducer connected tothe screen to break up agglomerates of the solid particles.
 16. Themethod of claim 1, wherein the first controlled secondary gas comprisesa first portion mechanically controlled by the at least one firstradially-symmetrical flame-stabilizing opening and a second portionactively supplied adjacent to the closed inlet end of the firstenclosure sidewall arranged to create turbulent flow in the firstenclosure.
 17. The method of claim 1, wherein the burner furthercomprises a flow restriction at the exhaust end of the second enclosuresidewall.
 18. The method of claim 17, wherein the flow restriction is inthe form of a reducer for introducing a tertiary gas.
 19. The method ofclaim 1, wherein: a gas feed system supplies the flow of the carriergas; a powder fuel feed system supplies the explosible powdered solidfuel; mixing the flow of the carrier gas and the plurality of solidparticles occurs in a mixing zone having inputs coupled to the gas feedsystem and the powder fuel feed system and an output comprising a movingstream of a dispersion of the solid particles in the carrier gas; and acontrol system coupled to the gas feed system and the powder fuel feedsystem regulates the flow of the carrier gas such that the moving streamslows to a flame speed of the moving stream at an equivalence ratio inthe explosible range downstream of the inlet nozzle to maintain a stablestationary flame front within the first enclosure.
 20. The method ofclaim 19, wherein the gas feed system supplies the second controlledsecondary gas and the control system further regulates the flow of thesecond controlled secondary gas from the gas feed system to achieve apredetermined level of combustion completeness of the explosible powderat the open exhaust end.
 21. The method of claim 19, wherein at leastone additional air inlet to the mixing zone provides an accelerating gasto accelerate the moving stream and break up agglomerates of the solidparticles prior to entry into the first enclosure.
 22. The method ofclaim 19, wherein the gas feed system comprises a pressurized gassource, a blower, or a gas amplifier device.
 23. The method of claim 19,wherein the powder fuel feed system comprises at least one fuel usagemeter.
 24. The method of claim 19, wherein the control system comprisesat least one sensor.
 25. The method of claim 24, wherein the sensor isselected from the group consisting of an intake temperature sensor, anexhaust temperature sensor, an intake gas sensor, exhaust gas sensor, amass airflow sensor, an air/fuel ratio sensor, a fuel flow sensor, anoxygen sensor, a carbon monoxide sensor, a powder supply sensor, anacoustic sensor, a powder sensor, a vacuum sensor, a pressure sensor, aposition sensor, a powder feed speed sensor, a static charge sensor, ahumidity sensor, a moisture sensor, a particle size sensor, an opticalsensor for sensing a presence of the flame, and any combination of thesesensors.
 26. The method of claim 19, wherein the gas feed systemcomprises a gas usage meter.
 27. The method of claim 19, wherein thepowder fuel feed system comprises an auger.
 28. The method of claim 19,wherein, for a preset BTU per hour output, the control system alsoregulates the first controlled secondary gas to maintain the movingstream at the equivalence ratio in the explosible range downstream ofthe inlet nozzle and to maintain the stable stationary flame frontwithin the first enclosure, wherein the first controlled secondary gasis actively provided by the gas feed system.
 29. The method of claim 19,further comprising filtering flue gases from the burner.
 30. The methodof claim 19, further comprising capturing and collecting unburned powderfuel or ash particulates exiting the open exhaust end of the secondenclosure in a collection vessel located downstream of the open exhaustend of the second enclosure.